Implement user-side module loading and module caching

[injuly]

Currently, only standard library modules can be loaded and cached.
We want to be able to load modules from anywhere within the current directory.

Note:

1. Calling import returns a table.
2. There are 2 kinds of modules
   2.1. Native modules written in C++ and exposed via the Vyse API.
   2.2. Modules written in Vyse (more common).
3. Native modules are a bit harder to deal with, because we have to make sure we keep a handle to the shared library around in memory. Loading the same module multiples must not create new handles, that counts as a memory leaks. Whether the handles should be garbage collected when the contents of a module die is still undecided.
4. Vyse modules are easier, we need to read some text from a file somewhere in the filesystem and the VM does the rest.

I feel further brainstorming on this topic will help come up with more concrete ideas (especially with 3).
This could be done in 2 PRs, one that loads vyse modules and one that loads native ones.

[injuly]

Closed thanks to #20 :D