yap

From #13:

• Add Tokens::parse method, which attempt to parse the remaining tokens into some type via FromStr (and str::parse). This requires a buffer type (often String) to be provided as the second generic arg, and may use this to buffer tokens prior to parsing if necessary.
• Add some helper functions which allow Tokens::parse to be optimised in cases like StrToken, so that eg calling toks.slice(from, to).parse::<u16,String> for StrTokens and similar won't allocate.
• Add Tokens::take for taking n tokens (can be useful in conjunction with parse, above).
• Add Tokens::eof, Tokens::collect and Tokens::consume for finding an EOF (None), collecting tokens and consuming all tokens.
• Add Tokens::into_iter, for when you need to work around lifetime issues with Tokens::as_iter.
• Rename Tokens::tokens_while to Tokens::take_while, and Tokens::skip_tokens_while to Tokens::skip_while; we no longer have to worry about conflicts with Iterator methods.
• Tokens::{surrounded, optional, optional_err} now accept an FnOnce instead of an FnMut, allowing a little more flexibility in what is given.
• Anything which returned something implementing Iterator now returns something implementing Tokens instead (you can still call .as_iter() to then get a standard Iterator from this).

A shoutout to @EasyOakland for their contributions in this!

From #13:

• Add yap::chars module which contains some helper functions (like parse_f64) specific to impl Tokens<Item=char>.
• Generalise yap::one_of! and Tokens::optional; the output from expressions passed to either of these can now be Option<T> or bool, or more specifically anything implementing yap::one_of::IsMatch.

Thankyou @Easyoakland for both of these contributions!

• Add support for no_std environments (#7).
• Add yap::types::IterToken, which can be used for parsing tokens from arbitrary iterators (as long as they impl Clone).

• Remove pointless skip_optional function.

• Tidy up some docs and deny missing docs on public items.

• Remove the Iterator supertrait requirement from Tokens, and instead require implementing those methods directly on Tokens. Add a Tokens::as_iter() method to return an Iterator from our Tokens. This is done to keep the Iterator interface and methods in a separate namespace from the Tokens ones.

• Fix a bug whereby .tokens("foo") will return true if we run out of input before completing the match.

• Add optional_err parser, which is like optional but allows any error to be propagated.

• Fix clippy warnings on one_of macro invocations.
• Run clippy on the codebase and add it to the CI.