Avoid StackOverflowError with erf* functions

[devmotion]

In the same way as #347, this PR avoids StackOverflowErrors and manually thrown MethodErrors by forwarding the erf* functions to an internal function (for which a MethodError is thrown if an implementation is missing).

Fixes #352, with this PR is a MethodError is thrown instead of a StackOverflowError:

julia> using SpecialFunctions, VectorizationBase

julia> vx = Vec(ntuple(_ -> 10randn(), VectorizationBase.pick_vector_width(Float64))...)
Vec{4, Float64}<-12.340876887492247, 12.779513587757672, -1.5552071156137923, -2.563055489830334>

julia> erf(vx)
ERROR: MethodError: no method matching _erf(::Vec{4, Float64})
Closest candidates are:
  _erf(::Float64) at /home/david/.julia/dev/SpecialFunctions/src/erf.jl:12
  _erf(::Float32) at /home/david/.julia/dev/SpecialFunctions/src/erf.jl:13
  _erf(::Float16) at /home/david/.julia/dev/SpecialFunctions/src/erf.jl:14
  ...
Stacktrace:
 [1] erf(x::Vec{4, Float64})
   @ SpecialFunctions ~/.julia/dev/SpecialFunctions/src/erf.jl:10
 [2] top-level scope
   @ REPL[6]:1

[codecov]

Codecov Report

Merging #353 (d086782) into master (c0442c2) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
+ Coverage   92.55%   92.58%   +0.03%     
==========================================
  Files          12       12              
  Lines        2712     2710       -2     
==========================================
- Hits         2510     2509       -1     
+ Misses        202      201       -1     

Flag	Coverage Δ
unittests	92.58% <100.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/erf.jl	100.00% <100.00%> (+2.04%)	⬆️
src/beta_inc.jl	89.47% <0.00%> (-0.31%)	⬇️

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