Fix equality testing of AST nodes with big.Int and big.Rat fields.
Merge pull request #33 from velour/bigint

Fix equality testing of AST nodes with big.Int and big.Rat fields.
Elide lint warnings about uncommented Eq methods.
Add pkgDecls.

It collects a mapping from package-scoped identifiers to their
declarations.
Add the scope type.
Add the Declaration field to Identifier nodes.
Add TestScopeFind.
Add a very incomplete Type.Identical implementation.

Currently, all methods panic except for (*TypeName).Identical, as it
is the only one needed for constant folding.
Start implementing Fold.