Original farrow implementation

[amir734jj]

Summary
This PR adds a synth-function evaluator based on the approach described in Farrow’s 1986 paper. It can be selected with -F0 or --synth-pure-farrow.

The evaluator generates a recursive function for each synthesized attribute. When it encounters a circular dependency, it evaluates the relevant dependency component repeatedly until its values stop changing.

What implementation supports:

• inherited and synthesized attributes
• circular attributes
• circular collection attributes attached to syntax-tree nodes
• local variables declared inside match rules
• circular local variables
• local collection variables
• conditional assignments and case expressions
• nested syntax-tree scopes
• non-fiber SNC grammars, such as simple-snc

Current limitations
Fibers are not supported. Since APS represents module-level dependencies using fibers, this also means the evaluator does not currently support:

• module-level global variables
• global collections
• circular global collections
• remote or object-field dependencies that require fibers

[boyland]

LGTM

[boyland]

Good changes