Consider making shank-idl sub-modules public

[buffalojoec]

Consider making sub-modules within shank-idl - such as idl_instruction - public.

Shank does a lot of the heavy lifting for you when it comes to parsing all of your Rust code into an IDL object, complete with serde.

If the rest of the modules were public, this would allow devs to build on top of Shank and customize their IDLs, perhaps by adding additional configurations for their client-side code.