cxx

First of all, great library!

I am having some confusion in understanding the role of values_first2 in set_difference_by_key as mentioned here.

In general terms, the result of set difference D = A - B will not contain values from B therefore values_first2 can never be part of D. So `val

• With rust 1.54 we can use the readme as docs.
• Rust 1.55 pattern match cleanups
• Replace the functions in unqualify.rs with a newtype wrapper for an unqualified ForeignItemFn suitable for use in extern "C'
• Rename FnAnalysis to FnAnalysisPhase, and FnAnalysisBody to FnAnalysis, and similar for the others
• Upgrade env_logger so that we don't end up need

There is some inconsistency in the documentation for cub::DeviceSpmv (https://nvlabs.github.io/cub/structcub_1_1_device_spmv.html). The overview says

The SpMV computation performs the matrix-vector operation y = alpha*A*x + beta*y,

But the function signature of CsrMV has neither alpha nor beta, and the function's description says

This function performs the matrix-vector 

We should provide a convenient way to pull in a rustup-like fashion, the remotely vetted toolchains.

Ref #53