Exact results for triangular functions

[hyrodium]

In the current master, the return value of triangular function is not exact.

julia> using Tau

julia> sin(τ)
-2.4492935982947064e-16

This can be solved like JuliaLang/julia#42595.

[devmotion]

This is not fixed yet.

[devmotion]

But should be fixed in IrrationalConstants now.