Build on Rust 1.21 stable

[gandro]

This commit removes the dependency on the unstable type_name intrinsic
and replaces it with the typename crate. As a result, we finally compile on Rust stable! This replaces #6.

The typename crate is our own alternative to named_type with the following differences:

• it implements its TypeName trait for all primitive and some std types. More types will be supported in the future, the current version however should be sufficient for all Timely/Differential programs we have written.
• It's output is compatible with the intrinsic and prints the full type name, i.e. not just std::vec::Vec, but std::vec::Vec<(string::string::String, i32)>

Fixes #4

[antiguru]

Looks fine

[gandro]

Thanks!