Remove Rmath sampling methods

[devmotion]

The ...rand methods in StatsFuns.RFunctions based on Rmath are not used in StatsFuns and not public API either. IMO if one wants to use these methods, it would be better to use Rmath directly (see JuliaStats/Rmath.jl#91).

The PR requires JuliaStats/Rmath.jl#91 and making use of Rmath in Distributions.

---

Additionally, to reduce the number of definitions further I removed the StatsFuns.RFunctions submodule and only defined the publicly exposed functions directly in the StatsFuns module.

[devmotion]

The PR is technically non-breaking (since the removed functions are not public API) but in practice breaks old versions of Distributions that make use of these non-public functions.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.39%. Comparing base (dee41f6) to head (5bf8720).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   70.88%   72.39%   +1.51%     
==========================================
  Files          17       22       +5     
  Lines         838     1007     +169     
==========================================
+ Hits          594      729     +135     
- Misses        244      278      +34     

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[andreasnoack]

I think this is good. Are there other breaking changes that we can come up with before releasing this?

[devmotion]

Maybe we could even think about removing the RFunctions submodule completely. With #191 interfacing with Rmath will not require complex ccalls anymore but we could instead directly define nbetapdf(...) = ... Rmath.dnbeta(...) etc within StatsFuns (possibly with the convenience matrix, possibly without).

[oscardssmith]

Maybe we could even think about removing the RFunctions submodule completely.

I would be very in favor (especially because I am going to try very hard to remove all remaining rmath dependencies over the next ~2 weeks)

[devmotion]

In its current form, the PR removes the submodule.