Test gamma corner cases
Apply tests to gamma instead of lgam
Move new_cmplx() to separate file cmath.lua
Add tests for cephes.gam()
Add tests for cephes.lgam()
Add alias cephes.digamma = cephes.psi
Add tests for cephes.psi()
Add cephes.polygamma()

Including tests and documentation
Add tests for cephes.igam()
Add tests for cephes.beta()
Update polygamma comment
Add cephes.betagrad() function
Add tests for cephes.fac()
Remove cephes default isnan and isfinite

These are covered by limits.lua, now.
Also removed spurious tests for these functions.
Add ffi include for cmath.lua

Stops tests failing due to ffi object no longer being global