add Indexed trait to compute generalized indices

[ralexstokes]

part I of adding multiproofs:

adds an Indexed trait so types can compute their own generalized indices from statically defined paths into each SSZ data structure

and pulls in earlier multiproof code from #2.

TODO:

• double-check implementation and things we support (e.g. no recursive custom types for now)
• port Container and Union implementations to proc derive macro

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -9.58 ⚠️

Comparison is base (e37de56) 75.29% compared to head (9977e20) 65.71%.

❗ Current head 9977e20 differs from pull request most recent head 21ae1a8. Consider uploading reports for the commit 21ae1a8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   75.29%   65.71%   -9.58%     
==========================================
  Files          18       21       +3     
  Lines         858      983     +125     
==========================================
  Hits          646      646              
- Misses        212      337     +125     

Impacted Files	Coverage Δ
ssz-rs/src/boolean.rs	77.77% <0.00%> (-13.53%)	⬇️
ssz-rs/src/lib.rs	100.00% <ø> (ø)
ssz-rs/src/list.rs	65.85% <0.00%> (-8.12%)	⬇️
ssz-rs/src/merkleization/mod.rs	93.10% <ø> (ø)
...src/merkleization/multiproofs/generalized_index.rs	0.00% <0.00%> (ø)
ssz-rs/src/merkleization/multiproofs/indexed.rs	0.00% <0.00%> (ø)
ssz-rs/src/merkleization/multiproofs/mod.rs	0.00% <0.00%> (ø)
ssz-rs/src/uint.rs	80.00% <0.00%> (-8.32%)	⬇️
ssz-rs/src/vector.rs	76.13% <0.00%> (-6.58%)	⬇️

... and 1 file with indirect coverage changes

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[ralexstokes]

something to check out: https://github.com/lightclient/proof

[ralexstokes]

closing in lieu of #120