Make more things public

[ekgame]

This change exposes a couple of struct fields for the public API. These are the fields I currently found useful for inspecting parsed code structure to reinterpret the code in a format that could be consumed outside Rust (with the goal to create a documentation generator).

The two fields are:

• Assembly#inputs - The library may load more than one file and it's useful to know what files were used.
• Function#signature - When inspecting a function it's useful to know it's inferred signature.

[kaikalii]

Function signature is private so that it cannot be changed. Use the getter.

[kaikalii]

Actually maybe it's fine.