Redesign import system.

[radian628]

As it currently stands, Desmoscript's import system is broken: Files cannot be imported without an accompanying namespace. Furthermore, using a file in multiple places leads to multiple copies of that file being instantiated. The simplest solution would be to use a C-style header/source distinction, but this leads to a lot of boilerplate and thus I will not pursue it.

I'll make several changes to fix the import system:

1. Don't check for duplicate identifiers (beyond ones in the same file), undefined identifiers, et cetera until after linking occurs.
2. Cache all imported files and compile them all in isolation.
3. Somehow find a way to make globally unique identifiers across multiple source files. This shouldn't be too bad once I am at the compile step and have all the linked files fully loaded.

I might have to rewrite the entire compiler from the ground up. This will also allow me to handle several poor design decisions, so it might be the best option. Everything past the AST phase (semantic analysis and compilation) will have to go, however.

[radian628]

Design changes to make:

• Use the visitor pattern instead of a huge switch statement for both semantic analysis and compilation, provided it doesn't add too much boilerplate.
• Generalize the idea of "mapping" over an AST node so I do not have to reimplement this functionality everywhere.
• Get rid of line/column/file info from AST nodes and instead make that part of the generic.
• Switch to nearley parser generator instead of ANTLR.