Have lib.rs re-export them, but make room for the upcoming Rust implementation.

This adds a `Script` trait that exposes slightly Rustier types in order
to have a common interface for the existing C++ implementation as well
as the upcoming Rust implementation (and a third instance that runs both
and checks that the Rust result matches the C++ one).

The module structure (interpreter.rs, zcash_script.rs) and locations of
definitions are intended to mirror the structure of the C++ code, especially as
we get the Rust implementation in place, for easier comparison. That
organization is very likely to change once everything has been checked.

Thanks to @nuttycom and @arya2 for getting the closure to work.

This should fix the Windows build.