Changeset 10776

Implement "nearly-lock-free" hash tables. All shared hash tables now
use this new algorithm (for more info see big comment in l0-hash.lisp).
To use the old hash tables, specify :lock-free nil as argument to
make-hash-table.

Incremented fasl version, in case there are any fasdumped hash tables out there.

While in there, changed defaults so that trying to write a read-only
table offers a restart to make it writable, with an appropriate warning.