Avoid CALL without RET for closure entries, hooks, and trampolines.

Avoid CALL without RET for closure entries, hooks, and trampolines.

This will wreck the CPU's return address branch target predictor.

This is an intermediate change en route to using paired CALL/RET for
continuation pushes and pop-returns in order to take advantage of the
CPU's branch target predictor.

WARNING: This changes the format of compiled closures, and as such,
new compiled code requires a new microcode and vice versa.