Lean verifiers + Hilbert implementation

[cheukhei-chu]

New files:

• mellea/stdlib/reqlib/lean.py: new verifies for Lean 4 code
• test/stdlib_basics/test_reqlib_lean.py: (incomplete) tests for some of the utility functions in lean.py
• docs/examples/hilbert/hilbert.py: new Hilbert class containing utility functions for constructing proof generation workflow
• docs/examples/hilbert/prove.py: customizable proof generation workflow using Hilbert class
• docs/examples/hilbert/mathlib_retriever.py: (unfinished, work in progress) new Retriever class which is an implementation of a semantic Mathlib4 theorem retriever

[mergify]

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