The `script` module is large and unwieldy.
Refactor the `script` module, splitting it up into a tree of modules.
Here are a few of the changes and their stated benefits
- Split the two script types out into separate files: Readers of the
methods can then tell immediately from the file name which type they are
reading.
- Put all the impls for the two script types together: Makes parsing the
API easier because one can more quickly see which traits are implemented
on what i.e., all the `AsRef` imlps are grouped together.
- Put the impls for the two script types in order, first `Script` then
`ScriptBuf`: Makes it easier for us to see if we missed something.
- Put the `Builder` and `Instruction` (and associated) types in their
own modules: Some devs find long files hard to navigate, so far there
hasn't been too much push back against short files.
- Put tests in a separate file: This idea was recently discussed.
This is only moving code and fixing import statements etc. No other
changes to the code.