Work in progress on in-language REPL

Includes some jester code for the REPL, and an implementation of a builtin for reading input. No implementation in the vm of eval yet, just lexer/parser/compiler so far. repl.jester compiles but crashes.

Add ?w=1 to the diff URL to ignore whitespace changes caused by tabs-spaces conversion in 4e18ccb. Would especially like your review on the input implementation in 6403d33.