fix(macros): `ty_nil` have been removed

See rust-lang/rust#18752
fix(lib): adapt to upstream changes

See:
japaric/simplot.rs#11
japaric/stats.rs#23
rust-lang/rust#18973
fix(test): arrays are no longer coerced to slices
Merge pull request #36 from japaric/fix

fix(lib): adapt to upstream changes