methprom.go covers method promotion.
Found bug: receiver() requires a following load under some
circumstances.
ifaceconv.go covers interface conversion.
Found bug: confusion about infallible and fallible conversions
led to use of TypeAssert in emitConv, which should never fail.
Changed semantics of ChangeInterface to make it infallible
and made some simplifications.
Also in this CL:
- SelectState.Pos now records the position of the
the '<-' operator for sends/receives done by a Select.
R=gri
CC=golang-dev
https://golang.org/cl/11931044
