compiler, runtime: Use function descriptors.

compiler, runtime: Use function descriptors.

This changes the representation of a Go value of function type
from being a pointer to function code (like a C function
pointer) to being a pointer to a struct.  The first field of
the struct points to the function code.  The remaining fields,
if any, are the addresses of variables referenced in enclosing
functions.  For each call to a function, the address of the
function descriptor is passed as the last argument.

This lets us avoid generating trampolines, and removes the use
of writable/executable sections of the heap.

From-SVN: r200181