Assume any external function won't let pointers live longer than the
call itself. This is true in the vast majority of cases (apparently
everywhere currently) but might not always be true.
TODO: add a //go:noescape (or maybe //go:escape) to handle this, instead
of this assumption.
This optimization makes sure the following pattern doesn't do a heap
allocation (assuming Write doesn't modify the slice):
var w *machine.UART = ...
w.Write([]byte("foo"))
As long as Write doesn't modify the slice and LLVM can detect this, a
call to runtime.stringToBytes with the necessary allocation + copy is
avoided.
