Improve support for additional load-paths

[emallson]

This PR was motivated by me wanting to create an AUR package for pixie, but being blocked by the hard-coded load-paths.

This PR contributes the following changes:

• load-paths are now added from the PIXIE_PATH environment variable when pixie-vm is run
• A single load path is added during build. This is intended for use by packagers to allow the standard library to be installed to a system directory such as /usr/share/pixie
• A make install target that more-or-less conforms to the variable conventions.

The order of the load-paths is [user path (-l), pixie-vm directory, PIXIE_PATH, system path].

Note that the pixie-vm path seems weird when doing a system install, because /usr/bin and a bunch of directories related to it end up in load-paths.

[heyLu]

Sorry for the late response...

While we need to support installing pixie separate from its libraries, I think we should do it "cleaner".

I'd prefer switching between the current path local to pixie-vm and a "system" path at build time. Either one or the other, but not both, because the current load path is confusing enough as it is.

Something like:

if USE_GLOBAL_LOADPATH:
  system_path = ... # from LIB_PATH or similar
else:
  system_path = ... # local to `pixie-vm`

[emallson]

Sorry for the also-late response :P

Understood. I'm going to close* this PR and use that approach. My reasoning for doing it this (messy) way was to preserve as much existing behavior as possible.

I should have a followup PR up this weekend.

*Closing because I don't see much point in having a big revert commit or overwriting that branch. Lemme know if you'd rather keep this centralized on this PR