peval: Handle optional argument inits that refer to previous arguments.

peval: Handle optional argument inits that refer to previous arguments.

Fixes <http://bugs.gnu.org/17634>.
Reported by Josep Portella Florit <jpf@primfilat.com>.

* module/language/tree-il/peval.scm (inlined-application): When inlining
  an application whose operator is a lambda expression with optional
  arguments that rely on default initializers, expand into a series of
  nested let expressions, to ensure that previous arguments are in scope
  when the default initializers are evaluated.

* test-suite/tests/peval.test ("partial evaluation"): Add tests.