Remove abstract fields

[devmotion]

This PR gets rid of the abstract fields by adding type parameters. It is a subset of the changes in #49.

A quick benchmark

using Bootstrap
using BenchmarkTools                                                                      
using StatsBase                              

function test_mean_and_std(n)                                                             
    r = rand(25)                                                                          
    sampling = BasicSampling(n)

    @btime bootstrap($mean_and_std, $r, $sampling)                                        
end                                          

function test_bca_conf_int(n)                                                             
    sample = bootstrap(mean_and_std, rand(25), BasicSampling(n))                          
    ci = BCaConfInt(0.95)                                                                 

    @btime confint($sample, $ci)
end                     

yields on the develop branch

julia> test_mean_and_std(100); 
  28.043 μs (8 allocations: 2.25 KiB)

julia> test_mean_and_std(1_000);                               
  287.129 μs (8 allocations: 16.38 KiB)      

julia> test_bca_conf_int(100);               
  29.124 μs (347 allocations: 33.13 KiB)     

julia> test_bca_conf_int(1_000);             
  134.613 μs (351 allocations: 47.53 KiB)    

and with this PR

julia> test_mean_and_std(100);     
  25.590 μs (5 allocations: 2.11 KiB)        

julia> test_mean_and_std(1_000);                                                          
  263.614 μs (5 allocations: 16.23 KiB)

julia> test_bca_conf_int(100);              
  12.927 μs (242 allocations: 30.81 KiB)

julia> test_bca_conf_int(1_000); 
  113.563 μs (242 allocations: 45.16 KiB)

[juliangehring]

Great, thanks!