Consistently make keyword-lists be static and read-only.

Consistently make keyword-lists be static and read-only.

We had an exciting mix of static and automatic storage durations, and
none were marked as `const'.  Python isn't `const'-correct, so we have
to cast away the `const'-ness: introduce a new macro `KWLIST' to do
this.

Also constify some other related tables, such as method names in
`pgev_python'.