Allow crate to compile without leaking features from dev-dependencies.

[najamelan]

When adding blanket as a dependency to a blank cargo project, it fails to
compile. This is due to missing features on the syn crate.

This commit adds the needed features. This problem could not be detected
from within the crate itself because features were leaking through from
the dev-dependency of syn which does not have default-features = false.

I turned on resolver = "2". This is the new feature resolver for cargo
that will become the default in edition 2021. This is not strictly necessary
but it helps as it no longer leaks features. This means that cargo check
will now fail if there are missing features. As cargo test obviously turns
on the features from dev-dependencies, this will still not detect the
issue.

You might want to check your CI script and add a cargo check before
running cargo test.

NOTE: rebased on #3 and #4, so you probably want to merge them first.

[codecov]

Codecov Report

Merging #5 (ba4ab9a) into master (c22888f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   78.18%   78.18%           
=======================================
  Files           9        9           
  Lines         495      495           
=======================================
  Hits          387      387           
  Misses        108      108           

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[najamelan]

ps I didn't run rustfmt, as that wanted to reformat the entire crate more or less.

[najamelan]

Ah, the merge conflicts caused a bug here. I'll try and fix.

[althonos]

Good catch, thanks!