Improve efficiency and speed of the macro expansion

[jmert]

This PR contains a number of code and performance improvements. (Commit f815fc1 by far dominates in the performance improvement.) In all, the time to generate bit flag definitions for a large-ish test case is reduced by about 35%.

The benchmarking script was generated with the code:

open("perf_many_bitflags.jl", "w") do fh                                                                                                                                                                                                      
    write(fh, "using BitFlags\n\n")                                                                                                                                                                                                           
                                                                                                                                                                                                                                              
    for ii in 1:16                                                                                                                                                                                                                            
        write(fh, "@bitflag InlineFlags$(ii)$(ii > 8 ? "::UInt64" : "")")                                                                                                                                                                     
        for jj in 1:3ii                                                                                                                                                                                                                       
            write(fh, " inlineflag$(ii)_$(jj)")                                                                                                                                                                                               
        end                                                                                                                                                                                                                                   
        write(fh, "\n\n")                                                                                                                                                                                                                     
    end                                                                                                                                                                                                                                       
                                                                                                                                                                                                                                              
    for ii in 1:16                                                                                                                                                                                                                            
        write(fh, "@bitflag InlineFlags$(ii)", ii > 8 ? "::UInt64" : "", " begin\n")                                                                                                                                                          
        for jj in 1:3ii                                                                                                                                                                                                                       
            write(fh, "    blockflag$(ii)_$(jj)",                                                                                                                                                                                             
                      ii % 2 == 0 && jj == 1 ? " = Int8(0)" : "", "\n")                                                                                                                                                                       
        end                                                                                                                                                                                                                                   
        write(fh, "end\n\n")                                                                                                                                                                                                                  
    end                                                                                                                                                                                                                                       
end

Benchmarking an include into an empty module on multiple version of Julia (keeping just the minimum) produced the following graph. (0.1.5 is the base release before the first commit, and 0.1.5-9-* is the last commit in the PR.)

[codecov]

Codecov Report

Merging #8 (be9a9eb) into master (3191eee) will increase coverage by 2.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   97.08%   99.34%   +2.26%     
==========================================
  Files           1        1              
  Lines         137      153      +16     
==========================================
+ Hits          133      152      +19     
+ Misses          4        1       -3     

Impacted Files	Coverage Δ
src/BitFlags.jl	99.34% <100.00%> (+2.26%)	⬆️

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