Add compilation profiles to std

[brson]

We would like the standard library to be compilable for multiple types of environments. Some environments (JS) don't like green threads, some (JS, microcontrollers) don't like threads at all, some (kernels) just need their own implementations of practically everything. Due to some complex interrelationships I don't think it's generally possible to factor all these capabilities out into their own crates, so we'll have to live with compiling std into different 'profiles'.

[bstrie]

Nominating for Well-Covered.

[catamorphism]

Just a bug

[emberian]

cc @cmr

[alexcrichton]

Closing, I believe that the libstd facade overrules this direction of making std more portable. Instead of having compilation profiles, we have chosen to have a "pick and choose" situation to select which crates work the best for your particular application.