Use weak pairs for some entries in obarray buckets.

Use weak pairs for some entries in obarray buckets.

Entries for symbols with global bindings still use ordinary pairs.
This enables interned symbols without global bindings to be garbage-
collected.  Operations in lookup.c that create or destroy global
bindings update the relevant obarray bucket entry to strengthen or
weaken it.

In the process, fix intern.c's string_hash to accept a long string
length, rather than a uint32_t string length, so that strings longer
than 4 GB will not lose on 64-bit systems.