Fix/core validation

[themighty1]

This PR adds validation to some of the core types.

Also renames tree_len to leaf_count since "tree length" is too ambiguous.

[codecov]

Codecov Report

Attention: Patch coverage is 76.27119% with 14 lines in your changes missing coverage. Please review.

Project coverage is 54.43%. Comparing base (2c045e5) to head (82e254c).

Files with missing lines	Patch %	Lines
crates/core/src/transcript/proof.rs	12.50%	7 Missing ⚠️
crates/core/src/transcript/encoding/proof.rs	84.21%	6 Missing ⚠️
crates/verifier/src/verify.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #648      +/-   ##
==========================================
+ Coverage   54.35%   54.43%   +0.07%     
==========================================
  Files         192      192              
  Lines       20474    20530      +56     
==========================================
+ Hits        11129    11175      +46     
- Misses       9345     9355      +10     

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