[dev.typeparams] go/*: switch from ListExpr to MultiIndexExpr

When instantiating a generic type or function with multiple type
arguments, we need to represent an index expression with multiple
indexes in the AST. Previous to this CL this was done with a new
ast.ListExpr node, which allowed packing multiple expressions into a
single ast.Expr. This compositional pattern can be both inefficient and
cumbersome to work with, and introduces a new node type that only exists
to augment the meaning of an existing node type.

By comparison, other specializations of syntax are given distinct nodes
in go/ast, for example variations of switch or for statements, so the
use of ListExpr was also (arguably) inconsistent.

This CL removes ListExpr, and instead adds a MultiIndexExpr node, which
is exactly like IndexExpr but allows for multiple index arguments. This
requires special handling for this new node type, but a new wrapper in
the typeparams helper package largely mitigates this special handling.

Change-Id: I65eb29c025c599bae37501716284dc7eb953b2ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/327149


Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>