fd_t: Refactor and fixes

[amyspark]

Hi @dragonmux,

This PR refactors fd_t to greatly reduce duplication, by using the readable_t, writeable_t and seekable_t interfaces you wrote. I also added attributes (nodiscard, inline) where relevant, and fixed the inheritance so that the whole set could interplay appropriately.

I've also taken care of the resize bug that @lethalbit reported.

Fixes #65

[codecov]

Codecov Report

Merging #68 (0052d85) into main (606e0cf) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
+ Coverage   92.62%   92.68%   +0.06%     
==========================================
  Files          48       47       -1     
  Lines        3390     3172     -218     
  Branches      610      608       -2     
==========================================
- Hits         3140     2940     -200     
+ Misses        193      175      -18     
  Partials       57       57              

Impacted Files	Coverage Δ
substrate/advanced/io	100.00% <100.00%> (ø)
substrate/fd	96.00% <100.00%> (-3.21%)	⬇️

... and 3 files with indirect coverage changes

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[dragonmux]

We've only reviewed the first half of fd_t so far as there are a lot of changes and we need to cross-reference the advanced IO types, but we figure we'd post the review early so we can discuss while we work on reviewing the rest of fd_t

[dragonmux]

LGTM, thank you for your excellent work on this! We'll get this merged once we've seen the builds go green.