Tweak features for regex-syntax

[alexcrichton]

This commit tweaks the features enabled for the regex-syntax crate
from the regex crate itself. This isn't intended to actually have any
functional change, but should help feature unification for Cargo in some
projects.

One project I work on exhibits an issue where executing cargo build
followed by cargo test will rebuild regex-syntax and all of its
transitive dependencies. The cause for this issue is that the tests are
using the proptest crate. The proptest crate depends on
regex-syntax with normal features (e.g. the defaults). All other
crates depend on regex with normal default features too.

The problem happens where when only the regex crate depends on
regex-syntax then the default and unicode features of
regex-syntax are disabled. This is because the regex crate disables
default features and regex's unicode feature delegates to all the
individual features of regex-syntax. When the regex-syntax crate is
depended on directly by proptest it then enables the default and
unicode features of regex-syntax.

Functionally these two builds of regex-syntax are exactly the same
since default is simply a proxy for unicode and unicode is simply
an umbrella including other features.

This PR updates the features enabled on regex-syntax by the regex
crate in two ways:

• The default feature for regex enables regex-syntax/default.
• The unicode feature for regex enables the regex-syntax/unicode
  feature.

This makes is so that if another crate in your crate graph depends on
regex-syntax then it'll have, by default, the same set of features
enabled than if you also depend on regex.

[BurntSushi]

@alexcrichton Wow, thank you for the detailed explanation and tracking this down! Definitely agree with this change, and this is probably how I should have set it up in the first place. :-)

[BurntSushi]

This PR is on crates.io in regex 1.3.7.

[alexcrichton]

Thanks so much! And no worries, it took me awhile to track this down b/c I've never had to do this to any other crates before either. A good tip for myself for the future though :)