Migrate to a native AST and Expr representation.

The original cel-go implementation was heavily based on protobuf definitions
for types, declarations, and expressions. While this was expedient, it has also
been cumbersome. With PR #568, the types and declarations were replaced
by Go-native structs. This issue tracks the migration of the Expr, SourceInfo,
and AST objects.

The motives for making this change are many:

• Performance
• Remove (most) hard dependencies on protobuf
• Allow for better interop between serialization formats and type systems