alloc prof: memoize stringifying type name

[vilterp]

Before:

95.639299 sec

After:

33.457880 sec

[codecov-commenter]

Codecov Report

Merging #48 (5b54ef3) into nhd-profile-allocs (df43527) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@                  Coverage Diff                   @@
##           nhd-profile-allocs      #48      +/-   ##
======================================================
+ Coverage               96.34%   96.39%   +0.04%     
======================================================
  Files                       3        3              
  Lines                     301      305       +4     
======================================================
+ Hits                      290      294       +4     
  Misses                     11       11              

Impacted Files	Coverage Δ
src/Allocs.jl	91.25% <100.00%> (+0.46%)	⬆️

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[NHDaly]

wow that's crazy. String construction is expensive! Thanks Pete - great idea 💪