Change flag storage to reduce memory use

[FollowTheProcess]

Summary

Changes Entry to *Entry in the internal flagset storage so that we can look up the same memory from shorthand or flag name.

Benchstat seems to suggest that although it allocates more (pointers rather than stack copies), the end result is actually faster
and uses 37% less memory when constructing a typical cli with cli.New

goos: darwin
goarch: amd64
pkg: github.com/FollowTheProcess/cli
cpu: Intel(R) Core(TM) i5-8257U CPU @ 1.40GHz
              │ before.txt  │             after.txt              │
              │   sec/op    │   sec/op     vs base               │
ExecuteHelp-8   7.910µ ± 1%   7.799µ ± 1%  -1.41% (p=0.000 n=20)
New-8           1.886µ ± 0%   1.741µ ± 0%  -7.69% (p=0.000 n=20)
geomean         3.862µ        3.685µ       -4.60%

              │  before.txt  │              after.txt               │
              │     B/op     │     B/op      vs base                │
ExecuteHelp-8   5.347Ki ± 0%   5.347Ki ± 0%    0.00% (p=0.001 n=20)
New-8           3.079Ki ± 0%   1.953Ki ± 0%  -36.57% (p=0.000 n=20)
geomean         4.057Ki        3.232Ki       -20.36%

              │ before.txt │              after.txt              │
              │ allocs/op  │ allocs/op   vs base                 │
ExecuteHelp-8   81.00 ± 0%   81.00 ± 0%       ~ (p=1.000 n=20) ¹
New-8           24.00 ± 0%   26.00 ± 0%  +8.33% (p=0.000 n=20)
geomean         44.09        45.89       +4.08%
¹ all samples are equal

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.42%. Comparing base (e355191) to head (dd56c69).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/flag/set.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
- Coverage   92.60%   92.42%   -0.18%     
==========================================
  Files           7        7              
  Lines        1028     1030       +2     
==========================================
  Hits          952      952              
- Misses         44       45       +1     
- Partials       32       33       +1     

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