Add runtime support for range over functions, specifically
for defer in the loop body. The defer is running in one
function but needs to append to the deferred function list
for a parent function. This CL implements the runtime
support for that, in the form of two new functions:
deferrangefunc, which obtains a token representing the
current frame, and deferprocat, which is like deferproc
but adds to the list for frame denoted by the token.
Preparation for proposal #61405. The actual logic in the
compiler will be guarded by a GOEXPERIMENT; this code
will only run if the compiler emits calls to deferprocat.