Add `builtin::numify`

[leonerd]

Adds a new builtin function, builtin::numify, the number equivalent of builtin::stringify.

While here also remembers to add a test and documentation on how stringify handles undef. No behavioural change to stringify itself.

[jkeenan]

@leonerd, this draft pull request has had merge conflicts for about 5 months. Do you intend to move forward with it? Thanks.