gc: five bug fixes, one better error.

* 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
diff --git a/test/assign.go b/test/assign.go
index ba80c8d..842bd62 100644
--- a/test/assign.go
+++ b/test/assign.go
@@ -1,4 +1,4 @@
-// errchk $G $D/$F.go
+// errchk $G -e $D/$F.go
 
 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -34,4 +34,12 @@
 		x = y;	// ERROR "assignment.*Mutex"
 		_ = x;
 	}
+	{
+		x := sync.Mutex{0, 0};	// ERROR "assignment.*Mutex"
+		_ = x;
+	}
+	{
+		x := sync.Mutex{key: 0};	// ERROR "(unknown|assignment).*Mutex"
+		_ = x;
+	}
 }