stat: Work around stack overflow in rustc

[kamalmarhubi]

Implement parsing of /proc//stat as a function instead of via the
chain! macro to limit the AST depth in expanded code. This works
around a stack overflow in some versions of rustc.

There is a performance impact, though a few hundred nanoseconds should
be acceptable in return for working on stable Rust.

name                  old ns/iter  new ns/iter  diff ns/iter  diff %
bench_stat            5,138        6,161               1,023  19.91%
bench_stat_parse      2,163        2,638                 475  21.96%

Benchmarks were run with nightly-2016-10-28, a recent nightly without
the stack overflow issue.

Fixes #9
Refs rust-lang/rust#35408

[kamalmarhubi]

This replaces #14. I think the implementation is cleaner, and the performance impact is slightly smaller.

[kamalmarhubi]

Ping :-)

[danburkert]

Thanks!