Improve the Base interface

[LPTK]

This is to prepare the ground for the new IR implementations that will replace the legacy AST/SimpleAST/SimpleANF IRs which are bloated and unoptimal.

We need to:

• add a Context abstract type member and an implicit parameter to reification methods, so as to avoid needless global state in the reification process;

• abstract over the representation of argument lists List[ArgList], so as to allow alternative and possibly more efficient representations;

• abstract over the Extract data type, for the same reason;

• perhaps take the precautions needed to make the IRs threadsafe, if it does not incur significant overhead (otherwise make it opt-in) – IIRC the main place where threadsafety would be a problem is with the pattern caching mechanism, and with variable-id generation.