spec: receiver declaration is just a parameter declaration

This CL removes the special syntax for method receivers and
makes it just like other parameters. Instead, the crucial
receiver-specific rules (exactly one receiver, receiver type
must be of the form T or *T) are specified verbally instead
of syntactically.

This is a fully backward-compatible (and minor) syntax
relaxation. As a result, the following syntactic restrictions
(which are completely irrelevant) and which were only in place
for receivers are removed:

a) receiver types cannot be parenthesized
b) receiver parameter lists cannot have a trailing comma

The result of this CL is a simplication of the spec and the
implementation, with no impact on existing (or future) code.

Noteworthy:

- gc already permits a trailing comma at the end of a receiver
  declaration:

  func (recv T,) m() {}

  This is technically a bug with the current spec; this CL will
  legalize this notation.

- gccgo produces a misleading error when a trailing comma is used:

  error: method has multiple receivers

  (even though there's only one receiver)

- Compilers and type-checkers won't need to report errors anymore
  if receiver types are parenthesized.

Fixes #4496.

LGTM=iant, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/101500044

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 28, 2014",
+	"Subtitle": "Version of June 24, 2014",
 	"Path": "/ref/spec"
 }-->
 
@@ -2029,13 +2029,14 @@ and associates the method with the receiver's <i>base type</i>.
 
 <pre class="ebnf">
 MethodDecl   = "func" Receiver MethodName ( Function | Signature ) .
-Receiver     = "(" [ identifier ] [ "*" ] BaseTypeName ")" .
-BaseTypeName = identifier .
+Receiver     = Parameters .
 </pre>
 
 <p>
-The receiver type must be of the form <code>T</code> or <code>*T</code> where
-<code>T</code> is a type name. The type denoted by <code>T</code> is called
+The receiver is specified via an extra parameter section preceeding the method
+name. That parameter section must declare a single parameter, the receiver.
+Its type must be of the form <code>T</code> or <code>*T</code> (possibly using
+parentheses) where <code>T</code> is a type name. The type denoted by <code>T</code> is called
 the receiver <i>base type</i>; it must not be a pointer or interface type and
 it must be declared in the same package as the method.
 The method is said to be <i>bound</i> to the base type and the method name