nsc: Rename nstr_exec_val() to nstr_eval() and tighten contract

nsc: Rename nstr_exec_val() to nstr_eval() and tighten contract

nstr_exec_val() can produce three different error values: NSC_NOTYPE
on invalid category, invalid type with zero val_as.lng on invalid type
(this is a bug), and the wanted type with zero val_s when it can't
coerce.  None of these should ever happen.

Fix it to always produce an NSC_NOTYPE error value.  Fix up callers to
check for it.

Specify the result's type is promoted on success.  Ensure it is even
when the argument is NSC_VAL with an unpromoted type, which is
invalid.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>