Handle rethrow

[timholy]

rethrow is only viable inside a catch, so we need to intercept it and convert it to an ordinary throw.

In my hands, this was perhaps the single biggest limitation of JuliaInterpreter. I frequently got

julia> @test ret == ErrorException("inner")
Test Failed at REPL[10]:1
  Expression: ret == ErrorException("inner")
   Evaluated: ErrorException("rethrow(exc) not allowed outside a catch block") == ErrorException("inner")
ERROR: There was an error during testing

I expect this to have a substantial positive impact on #13.

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 0.61%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   70.81%   71.42%   +0.61%     
==========================================
  Files           6        6              
  Lines        1124     1127       +3     
==========================================
+ Hits          796      805       +9     
+ Misses        328      322       -6

Impacted Files	Coverage Δ
src/interpret.jl	71.22% <100%> (+1.93%)	⬆️

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