[Ur] Request environment variables

[Adam Chlipala]

Environment variables are now available via [Basis.getenv], and you can 
produce properly typed arguments for it with [Basis.blessEnvVar].

I ended up not implementing the abstract [http] type that I proposed, 
because it was unclear how to guarantee that such a value is always 
available, without messing with the way that intra-application links are 
encoded/typed.