Initial Rust implementation

[sellout]

This provides two more implementations of the ZcashScript trait. One is a pure Rust implementation and the other runs both the C++ and Rust implementations, returning the C++ result and logging a warning if they differ.

This follows the C++ implementation extremely closely for ease of initial review. Future changes will move from this to a Rustier approach.

E.g.,

• early returns
• asserts
• bounds checking followed by accesses¹
• if instead of match
• mutable refs
• etc.

It differs from the C++ in some places, as well. Numeric types and casting/From are changed in some places to avoid partiality. Also, the Opcodes are partitioned into multiple types because we eventually want to be able to enforce push_only at the type level, and PushValue operations carry data (although not yet in the structure), which makes them distinct from operations (PushdataBytelength, in particular).

Footnotes

1. accesses generally return in Result, so ? ensures they’re safe, but we keep the bounds checking so that the stack state matches the C++ version (e.g., without the bounds checking, an op that pops two elements off the stack could fail after popping one, which would leave the stack in a different state than the C++ impl which checks the length of the stack before popping either). ↩

[sellout]

Superseded by #174, which is based on a branch on this remote, for easier chaining of PRs.