Bundle FFTW source code with fftw-src

[hombit]

Downloading the source code of FFTW on every compilation could be inefficient or even impossible in some environments. For example, network access is blocked on docs.rs, that caused fftw-src 0.3.1 docs build fail, and can cause similar problem for dependent crates. This is why I propose to include FFTW source code into fftw-src crate.

[LingMan]

That would also get rid of the reqwest dependency which takes a significant amount of time to compile by itself. +1

[termoshtt]

I close this because #88 have realized source code bundling. Downloading issue is inherited into #89