Attach trailing trivia to tokens

[JohnnyMorganz]

As a follow up to #71, this PR starts adding trailing trivia to tokens.

Trivia on a token is split up into leading and trailing trivia. Leading / trailing split is based on the Roslyn Compiler trivia rules: trailing trivia consists of everything up to and including the first \n newline character, and all further trivia is leading trivia of the next token. The idea being that local x = ...\n can then be represented as a single line in trivia, to allow easier replacement (rather than the \n being part of the next statement).

This is done by:

• Storing a reference to the previously serialized token
• When serializing a new token, split the extracted trivia into trailing trivia of the previous token, and leading trivia of the next token
• Attach trailing trivia onto the previous token (via the stored reference lookup)