Add the ability to run on no_std targets

[dflemstr]

A new attempt at #30, this time realizing that allocation is probably necessary for the algorithms to work.

[coveralls]

Coverage increased (+0.02%) to 97.32% when pulling de272fa on dflemstr:master into 8d295f4 on chalharu:develop.

[dflemstr]

I will leave this open until the 0.2.8 release of num-traits which contains a required feature

[chalharu]

It's a good idea, but I assume that need to prevent libm from being added to the std target

[dflemstr]

Hmm you are right, not sure how that can be done easily... I'll try to think of something

[dflemstr]

This is now using the released version of num-traits which contains the libm feature. I also ran cargo upgrade to ensure that semver is properly followed for the dependencies and avoid build failure when some other library would introduce a wrong version for chfft.