[NFC] Split responsibilities of Mapper

[lucic71]

Mapper grew too big. It has 4 responsibilities at the same time:

1. Map expressions or types to a rust string
2. Match expressions or types against translation rules
3. Answer queries about the translation rules (ParamIsPointer, IsLibcPassthrough, etc)
4. Print string representation of expressions or types

Only the first should be in Mapper, the rest should be split.

For (2) I plan to add a new tree matcher instead of the current string matcher. To make the transition smooth I extracted the matching logic form Mapper into cpp2rust/converter/rules/matcher.h.

For (3) I added cpp2rust/converter/rules/registry.h which keeps the exprs_ and rules_ multimaps + handles queries about the translation rules.

For (4) I added cpp2rust/converter/printer.h which should BTW disappear after we switch to tree matching instead of string matching.

This is a building block for fixing #435