Use IrrationalConstants and sinpi/cospi

[devmotion]

This PR simplifies the package and makes it more consistent with base Julia and the ecosystem. It

• defines tau (and its unicode alias) as alias of IrrationalConstants.twoπ instead of a new irrational constant,
• defines modtau as alias of mod2pi instead of a new function,
• defines sintau and costau as sintau(x) = sinpi(2 * x) etc. to make use of the implementations in https://github.com/JuliaLang/julia/blob/master/base/special/trig.jl without duplicating them and having to synchronize them,
• defines sintau(x::Integer) = zero(float(x)) and costau(x::Integer) = one(float(x)) instead of zero(x) and one(x) to be consistent with base Julia (https://github.com/JuliaLang/julia/blob/fc9c280584f63236a1b97da4178a41eba65b6da2/base/special/trig.jl#L935-L936 and https://github.com/JuliaLang/julia/blob/fc9c280584f63236a1b97da4178a41eba65b6da2/test/math.jl#L597),
• adds CompatHelper,
• fixes the tests (currently they include the source file instead of actually loading the package).

By using aliases one gets all existing functionality for twoπ and mod2pi for free.

Intentionally I did not fix JuliaMath/IrrationalConstants.jl#20 and #39 to avoid introducing any new functionality and definitions.

The main user-facing change is the re-definition of sintau(x::Integer) and costau(x::Integer) and that tau isa IrrationalConstant{:twoπ}. I'd argue that the former is a bugfix but maybe it is safer to make a breaking release, also due to the type change. Probably it does not matter though since the only direct dependent of Tau.jl 0.2.0 was PredictMDExtra (https://juliahub.com/ui/Packages/Tau/jo2zN/0.2.0?page=2) which however does not depend on Tau anymore (https://github.com/bcbi/PredictMDExtra.jl/blob/master/Project.toml, changed in 2019: bcbi/PredictMDExtra.jl#149).

cc: @hyrodium

[codecov]

Codecov Report

Merging #40 (c3f187d) into master (4216830) will increase coverage by 2.94%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       JuliaMath/Tau.jl#40      +/-   ##
===========================================
+ Coverage   97.05%   100.00%   +2.94%     
===========================================
  Files           2         1       -1     
  Lines          68         4      -64     
===========================================
- Hits           66         4      -62     
+ Misses          2         0       -2     

Impacted Files	Coverage Δ
src/Tau.jl	100.00% <100.00%> (ø)

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[giordano]

Looks good to me, thanks!