Add ABI annotations to function signatures.

Specify the location of arguments as well as the size of stack argument
array needed. The ABI annotations are optional, just like the value
locations.

Remove the Eq implementation for Signature which was only used by a
single parser test.

docs/langref.rst

@@ -893,6 +893,11 @@ Number of arguments to a function
     incoming frame pointer, and callee-saved registers that are saved in the
     prologue.
 
+Size of function call arguments on the stack
+    At most :math:`2^{32} - 1` bytes.
+
+    This is probably not possible to achieve given the limit on the number of
+    arguments, except by requiring extremely large offsets for stack arguments.
 
 Glossary
 ========