Allow constant-only expressions in source

[davemenendez]

At present, the following is disallowed, because expressions are not permitted in the source:

%1 = or undef, 1
%2 = and undef, ~1
%3 = add %1, %2
%4 = and %3, 1
 =>
%4 = 1

This could be resolved by

1. Adding initial ~ to the syntax for numeric literals.
2. Permitting constant expressions which contain no inputs in the source. This could be done by folding expressions to constants during the parse action.

Adding general support for expressions in the source is not recommended, as this makes the C++ translation too complex.

[davemenendez]

On further thought, it should be feasible to allow constant expressions in the source as long as they don't introduce new variables.