change: separate out tls feature flags

[Fishrock123]

A successor to #53, this separates out the tls feature flags, which should be nicer.

Fixes: #54

[Fishrock123]

@JEnoch sorry for being slow, does this look good to you?

[JEnoch]

@Fishrock123 yes, looks good to me!
But I think the user doc shall make clear that using native-tls or rustls with something else than h1_client is probably not a good idea, right ? (or at best has no effect)

[taiki-e]

This seems to break when both "native-tls" feature and "rustls" feature are enabled, or when both "native-tls" feature and "rustls" feature are disabled.

• "native-tls" feature and "rustls" feature are new features in 6.3, so this breaks all crates that using the "h1_client" feature and disabling the default features. (e.g., surf)
• The cargo feature is shared across the dependency graph, so if both features are specified somewhere in the dependency graph, this breaks the build.

See also cargo reference: https://doc.rust-lang.org/nightly/cargo/reference/features.html

[taiki-e]

I filed a fix to surf (http-rs/surf#282), and I plan to send a similar patch to http-client later.

EDIT: filed #69 to fix http-client