fix common parameter cancelation

[MikaelSlevinsky]

add terminating check
add generalized euler transformation (in conformal rational approximation)

Before, with @TSGut 's common parameter cancelation:

julia> using HypergeometricFunctions, BenchmarkTools

julia> α = (3,4,5)
(3, 4, 5)

julia> β = (2,6,7)
(2, 6, 7)

julia> z = 0.123
0.123

julia> @time pFq(α, β, z)
  0.253318 seconds (363.21 k allocations: 22.849 MiB, 10.90% gc time, 99.98% compilation time)
1.091834662493663

julia> @btime pFq($α, $β, $z)
  1.605 μs (25 allocations: 624 bytes)
1.091834662493663

julia> α = (1,1,1,1,1,3,4,5)
(1, 1, 1, 1, 1, 3, 4, 5)

julia> β = (1,1,1,1,1,2,6,7)
(1, 1, 1, 1, 1, 2, 6, 7)

julia> @btime pFq($α, $β, $z)
  3.035 μs (50 allocations: 1.22 KiB)
1.091834662493663

After this PR, performance is somewhat restored:

julia> using HypergeometricFunctions, BenchmarkTools

julia> α = (3,4,5)
(3, 4, 5)

julia> β = (2,6,7)
(2, 6, 7)

julia> z = 0.123
0.123

julia> @time pFq(α, β, z)
  0.463688 seconds (1.07 M allocations: 68.306 MiB, 5.75% gc time, 99.99% compilation time)
1.091834662493663

julia> @btime pFq($α, $β, $z)
  76.297 ns (0 allocations: 0 bytes)
1.091834662493663

julia> α = (1,1,1,1,1,3,4,5)
(1, 1, 1, 1, 1, 3, 4, 5)

julia> β = (1,1,1,1,1,2,6,7)
(1, 1, 1, 1, 1, 2, 6, 7)

julia> @btime pFq($α, $β, $z)
  317.771 ns (32 allocations: 1.19 KiB)
1.091834662493663

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 105 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.61%. Comparing base (958bae6) to head (6a76550).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/conformal.jl	0.00%	96 Missing ⚠️
src/specialfunctions.jl	86.88%	8 Missing ⚠️
src/generalized.jl	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
- Coverage   87.30%   80.61%   -6.69%     
==========================================
  Files           8        8              
  Lines        1796     1945     +149     
==========================================
  Hits         1568     1568              
- Misses        228      377     +149     

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[TSGut]

Nice way around the tuple instability!