feat(buffer): add capacity function to memory and span buffer

[Viatorus]

Fixes #85

For other buffers types the capacity isn't evident.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (46cfdf5) 99.38% compared to head (64f4363) 99.38%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          58       58           
  Lines        6032     6044   +12     
=======================================
+ Hits         5995     6007   +12     
  Misses         37       37           

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