transl: allow rest args and improve validation in MQAPPLY lambda forms

transl: allow rest args and improve validation in MQAPPLY lambda forms

foo would fail to translate due to the rest arg:

    foo () := lambda ([[x]], x) (5)

but things like bar and baz would translate and work just fine:

    bar () := apply (lambda ([[x]], x), [5])
    baz () := block ([f : lambda ([[x]], x)], f (5))

Now rest args are allowed in MQAPPLY lambda forms.

Validation of lambda expressions (checks for an empty body, duplicate
parameters, etc.) in lambda forms has now also been improved.

No problems with the test suite, share test suite or rtest_translator.
New tests have been added to rtest_translator.