let-values splicing faulty in interpreter

[fare]

The implementation of let-values in src/gerbil/runtime/gx-gambc2.scm fails to support rest arguments in the values list, as in (let-values (((a . b) (values 1 2 3))) [a b]).

This notably prevents an (import :std/srfi/43) from succeeding in the interpreter during bootstrap, but is still generally an issue with let-values in the interpreter beyond bootstrap.