Support STABLE functions in INSERT statements

We took a conservative approach in function evaluation for `INSERT`: we evaluate immutable expressions (such as `2 + 2` or `char_length('postgres')`) during planning but reject any other expressions.

This means certain idioms (like using `now()` to timestamp a row) are unavailable when using `pg_shard`. The next logical step would be to coerce `STABLE` functions into constants during `INSERT`: evaluating them on the master would ensure all replicas receive an identical value.