* check for struct literal assignment to private fields.
* record, fix crash involving parallel map assignment.
* avoid infinite recursion in exportassignok.
* make floating point bounds check precise.
* avoid crash on invalid receiver.

* add context to error about implicit assignment.

Fixes #86.
Fixes #88.
Fixes #158.
Fixes #174.
Fixes #201.
Fixes #204.

R=ken2
https://golang.org/cl/154144