Add static "from" methods and allow writing to files

[LeopoldArkham]

• Parser::new() should become Parser::from() and accept anything that AsRef's to &str
• Parser::from_file() should be added and accept anything that AsRef's to Path
• TOMLDocument::save_to() should be added and accept anything that AsRef's to Path
• TOMLDocument::save() should be added, it overwrites the source file if the parser was created through the Parser::from_file() method and returns an error otherwise.

The tests will need to be updated to use these new methods

[markcol]

Taking #5 in mind, should Parser::::from() automatically parse the data? For example, the test in parser.rs line 757, could look like either of these::

let _ = Parser::from(text).parse();

or

let _ = Parser::from(text);

The later feels better/more ergonomic to me.

If you want to make all of the Parser functions private, you could add a from() and from_file() onto the TOMLDocument, and then have the document control when parsing happens. It depends on what shape the API would be for the users.

[LeopoldArkham]

Actually, from() and from_file() could even be free functions, that's how most parsing libraries seem to do it:

extern crate molten;

let doc = molten::from_file("cargo.toml");

Seems like the best solution to me!

[markcol]

I agree. I'm working on it.

[LeopoldArkham]

Great! Note that from_file may have to become from_reader, otherwise the references in the parsed document will outlive the original file