Bump to 2018, add optional 2nd arg, use spans and quote, add wrap macro for stable rust

[TheButlah]

This PR seeks to improve this crate with the following:

• Bumps edition to 2018 to use more modern macro import style
• Uses spans to improve error messages
• Uses the quote! crate instead of manually constructing token streams
• Adds an optional 2nd argument for dynpath macro that gives the ability to manually change the suffix
• Adds a wrap macro that lets the dynpath macro work on stable rust
• Improves documentation of the macros
• Doctests no longer fail

Closes #1

[TheButlah]

Pinging reviewer 👀