feat: Add Rust bindings target

[samuelburnham]

• Adds Rust bindings for Blake3 using https://github.com/argumentcomputer/lean-ffi to construct and manage lean.h objects
• Adds a common HasherOps typeclass that both C and Rust hashers implement
• Refactors the lakefile to allow for independent build targets based on the user importing Blake3.C or Blake3.Rust, neither interferes with the other due to the use of target instead of extern_lib
• Updates Blake3Test.lean to allow for testing C or Rust separately with lake test -- <c|rust> or both by default
• Adds a Valgrind memory leak test in CI

Usage example: argumentcomputer/ix#329

[arthurpaulino]

Great! I'm approving given the fact that argumentcomputer/ix#329 works 👏🏼