Switch to using LEB encoding for key and data lengths in on-disk hash

tables.

This gives a modest AST file size reduction, while also fixing crashes
in cases where the key or data length doesn't fit into 16 bits.
Unfortunately, such situations tend to require huge test cases (such as
more than 16K modules or an overload set with 16K entries), and I
couldn't get a testcase to finish in a reasonable amount of time, so no
test is included for that bugfix.

No functionality change intended (other than the bugfix).