eval: fix optional parameter bug from 2014.

This bug affects optional parameters which either
have no default expression, or one that is nil.
For instance x in (lambda (: (x nil))) or (lambda (: x)).
When such a parameter is given the : symbol as an argument, it
is not being bound, as if it weren't there.

((lambda (: x) x) :) -> ;; error: unbound variable x

This issue is not a regression; it was introduced in the
commit which introduced the colon convention to optionals, as
well as init expressions and presence-indicating variables,
commit 68c084269581f32f0a7b859446ae2efb6c6a26c0 made in
February 2014.

This might be the first instance of an interpreter bug being
found that is not present in the compiler.

* eval.c (bind_args): The idea here was that when the argument
to an optional the colon keyword symbol, and the optional's
initform is nil, we can skip the overhead of calling eval to
get that initform's value. Unfortunately, the skip was
extended over the code which binds the parameter. Only
the eval can be skipped!

* tests/012/lambda.tl: New test cases to cover this.

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