Add Script builder

[gakonst]

It'd be nice if instead of having to write scripts as strings manually, I could have a builder object which would expose a type-safe (as much as typed-python allows) interface for creating scripts.

Example from rust-bitcoin:

• Definition: https://github.com/rust-bitcoin/rust-bitcoin/blob/master/src/blockdata/script.rs#L557-L637
• Test: https://github.com/rust-bitcoin/rust-bitcoin/blob/master/src/blockdata/script.rs#L810-L818

[prestwich]

I like the pattern, but don't have an urgent need. Existing tools can produce (almost) any Script