spec: reject top-level init names

Fixes issue 4586.

R=golang-dev, remyoudompheng, r
CC=golang-dev
https://codereview.appspot.com/7307082
spec: document fp rounding during explicit constant conversion

The gc compilers already behave this way. It was an explicit
decision during the very long constant design discussions,
but it appears not to have made it into the spec.

Fixes issue 4398.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7313069
spec: only require parens around ambiguous conversions

This is documenting the status quo. The previous cleanup
added this language as an implementation restriction, but
at least for now it is really part of the language proper.

Fixes issue 4605.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7305071