Add no_std + alloc support by way of a default "std" feature flag

[ZackPierce]

What

The addition of a feature flag, and conditional compilation blocks to make utf8-ranges work in either std or no_std + alloc environments, very much in the same vein as BurntSushi/aho-corasick#28 .

The default-on "std" feature flag controls whether or not to compile in #![no_std] mode. As stated in the updated docs, due to the use of the presently-unstable alloc API, no_std mode only works with a nightly toolchain at present.

How

To build for std, nothing changes. cargo build

To build for no_std:

cargo build --no-default-features --features "alloc"

The test suites pass as normal in either mode.

Why

This PR is part of an effort to get regex and its dependencies to operate in no_std + alloc mode.