Support for no_std

[m4rw3r]

Support for using Chomp without Rust's standard library, instead relying solely on the core module.

• types::Buffer::to_vec and types::Buffer::as_vec will not be a part of Chomp when built with core.
• buffer module will require some refactoring to allow for usage without std::io, and the buffer for the fixed-buffer needs to be user-supplied (because we can't allocate ourselves). See https://github.com/whitequark/rust-log_buffer for a good example on how to use user-provided buffers.
• parsers::Error cannot implement std::error::Error

[m4rw3r]

Decided to skip the buffer module for now, #57 should hopefully enable at least limited helper functionality for buffers without requiring std.

[m4rw3r]

Does not play well with examples since they require std. And cargo doesn't seem to have any feature which enables examples to depend on a feature defined in Cargo.toml. Temporarily renaming the examples/ directory when running cargo test works but is suboptimal, and adding feature-tests in examples is also a bad idea since they require certain features to function and #[cfg(feature="std")] all over the place would be distracting and not representative of the normal usage of Chomp.

[m4rw3r]

When rust-lang/cargo#1570 has been resolved we can also add --no-default-features to automatic tests. Until then manual testing is required.