Add new API: amount (aka length-ish)

[tkf]

No description provided.

[codecov-io]

Codecov Report

Merging #22 into master will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   92.45%   92.79%   +0.33%     
==========================================
  Files           3        3              
  Lines         106      111       +5     
==========================================
+ Hits           98      103       +5     
  Misses          8        8              

Impacted Files	Coverage Δ
src/SplittablesBase.jl	100.00% <ø> (ø)
src/implementations.jl	89.61% <100.00%> (+0.42%)	⬆️
src/testing.jl	100.00% <100.00%> (ø)

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