Make `UnexpectedCharacters` print the repr

This is an additional response to #738.

I also often came across this when unexpectedly having a newline, which would previously result in the message being split up across two lines. This PR makes it easier to identify exactly which special character is causing problems.

Fix a typo

Resolved conflicts manually /erez

Add documentation on the `x` regexp flag

Multiline regexps were added in https://github.com/lark-parser/lark/pull/645

cache=True now uses a temp directory instead of working directory

make _all_terminals a dict
added raw attribute to Pattern
rename nice_print -> user_repr

rename LexerConf.terminals to LexerConf.tokens
Make Exception message generation lazy
Made a few classes new-style