WIP: C++ Vctr class

[krlmlr]

• Currently implemented: logical, and a default class that forwards to base R (in inst/include/detail)
• enum VctrTypes; one function get_type() that maps a given SEXP to an enum value, after that everything is done with template logic
• Aimed towards extensibility, coercion logic specified via traits in inst/include/traits

PR branch: https://github.com/krlmlr/vctrs/tree/f-classes-draft

Currently a very rough draft, still missing:

• tests
• more elementary data types
• factors
• date/time
• performance tests
• a Vctr implementation that allows registration of Vctr classes at run time

@lionel-: Happy to take your feedback anytime.

Addresses #7 and #27.

[hadley]

I'm going to close this for now, since I think we have a better feel for how the implementation will work.