Features from optional dependencies

heck implicit feature

Enables heck ^0.4.0

heck:

heck is a case conversion library

http implicit feature

Enables http

http:

A set of types for representing HTTP requests and responses

Affects const-str-proc-macro::verified_header_name

regex implicit feature

Affects const-str-proc-macro::verified_regex, const-str-proc-macro::regex_assert_match