Rename std/crypto to std/hash

[brianp]

The std/crypto module name is misleading. The module only provides sha256, which is a hashing function. The name suggests a broader cryptography library, or worse, gets associated with web3 nonsense.

Rename to std/hash. If HMAC or key derivation functions are added later, std/digest is another option (Ruby's convention), but std/hash covers the current scope.

Changes needed:

• Rename codegen/src/runtime/crypto.rs to hash.rs
• Update runtime_sigs.rs (function registration)
• Update typechecker.rs (module registration, std/hash { sha256 })
• Update aster-pkg/src/main.aster (use std/hash { sha256 })
• Update any tests that import std/crypto