Refactor ffi impls into com.oracle.truffle.r.ffi.impl project; add ffi AP; TruffleLLVM all RFFI interfaces implemented; no JNI

coalesce all seq builtins into SeqFunctions
add fast paths for seq and seq.default
refactor and expand seq unit tests

- code corresponding to 'nmath' in GnuR moved to new package:
com.oracle.truffle.r.runtime.nmath
- it turns that Wilcox uses Choose, which is a builtin node, the original
structure did not allow code reuse in such cases
- implement wilcox externals {d/p/q}wilcox and wilcox_free + improved choose

Externals implemented: {p/q/d/r}nbinom, and {p/q/d}nbinom_mu