Clean up the interpreter's source, tests, documentation, etc.

[FascinatedBox]

The inner code has become a pretty big mess of things. Here are some basic issues:

• There are too many giant comments that are simply oversized. Some of these are relics from when I wrote them and was only half-understanding things.
• It should be easier to add keys to the symtab's builtins.
• Better documentation for all the important modules.
• Audit all of functions to determine what should stay and what should be shuffled around and such.
• Some functions have long names and shouldn't.
• Functions ought to have something where it's easy to figure out where they come from too. lilyS for symtab, lilyE for emitter, etc.
• Better, nicer documentation for the overall part of the language. Here, I intend to use Wren as an inspiration.
• Some documentation is old too. I'll bet that the opcode stuff in lily_opcode.h mentions nil-ity which is no longer relevant.
• One nice idea (I think) is to have a documentation page which explains the basics of how the interpreter accomplishes a certain task. Design documents, so to speak. Because some of what the interpreter does is kind of complicated (especially when generics and enum classes get into the mix).
• Some of the new ideas will get pulled into this commit as well. 'define' for defining functions is one that I see as a must.
• Tests should be organized by what they test. Again, borrowed from Wren.

The above isn't intended to be all-inclusive. My belief is that the work spent in cleaning up the core will pay dividends later on in the ability of others to be both willing and better able to contribute to the language.

This will be the last commit of this release. I would like the next version of Lily to have a clean slate insofar as the documentation being up-to-date, the two major changes to code style being done, etc.

[FascinatedBox]

93, the thing about type declarations, will be merged in-between various commits related to this particular issue. I don't want the first release of a new version to have a huge breaking syntax change.

[FascinatedBox]

I'm calling this done...any more effort spent cleaning up things looks like it will have diminishing returns.

As far as a website goes, I intend to do a small website as part of a final project for one of my classes (but it may be a while before the thing is done though).

For now, the tidying up that I really wanted to do is done.