Add composite primary keys

[elcritch]

• Add support for composite primary keys and foreign keys
• fixes postgres tests in CI (!)
• fixes postgres compilation on MacOSX for brew users
• adds script and task to setup postgres for local testing
• update docs some
• swap std/re for std/peg to remove libpcre deps
• tests run on PR's

[elcritch]

@Araq Yea / nea?

[Araq]

yea.

[elcritch]

yea.

LGTM then! Having postgres tests is nice.