Parser

[susan-garry]

Integrate a bunch of new parser features! Things that didn't parse before but now do include:

• 2-tuples
• Record initialization and field access (e.g., ('RecordType { f1 = v }).field1and{r1 with f1 = v2}`)
• Control flow (while, for, if/elif/else)
• Object declaration (Strand())
• Function and method calls (fun();, strand.push('A');)
• Set types (Set<t>)
• emit-to statements
• Function definition
• parset and graph declarations
• import statements

Other changes:

• Add a testing framework using runt
• Types go after identifiers (int i -> i: int)
• Add tests for depth.pollen and crush.pollen (I chose .pollen as the pollen file extension because .pol is apparently already a thing, but if no one here has heard of it then it's probably safe to make .pol the cannonical file extension).