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

- use RMathError
- fix format String in debugging messages
- move methods from GammaFunctions to RMath
- fix javadoc in QuantileSearch

getCanonicalFile() removes any '..' from the path. Some IDEs (intellij) put
'..' into StandardLocation.SOURCE_OUTPUT. With '..' the getParentFile() does
not work as intended.

+ small fix in RErrorHandling: when max number of warnings is reached, do not
report the actual number, but the max (like GnuR does).

Provide parameter values for each distribution separately +
honor semantics of common parameters like probability in [0,1] etc.

Few minor fixes of bugs discovered along the way.

Plus 'pgamma' external implemented.

+ necessary DPQ/RMath methods
+ move all XChisq to Chisq.java