Implement vars, choose/when, and expressions

[tyler]

• Basic variable assignment (esi:assign)
• Short form esi:vars
• Long form esi:vars
• Expression parsing and evaluation
• esi:choose & esi:when
  • parse
  • interpret
• matches operator
  • also matches_i
  • implement regex matching
  • expose captures (requires map vars)
  • respect matchname parameter
• triple tick strings
• minimal stdlib
  • function calls
  • implement library calls
    • lower
    • html_encode
    • replace
• map variables
• request metadata
  • host, method, querystring
  • headers in general? (what else is needed here in the short term?)

Cleanup tasks

• revamp the parser to identify expressions in raw text (I think this is ultimately a separate parser from the expression parser. They're similar but not identical.)
• identify function calls correctly (i.e. $fn($(var), ...) is valid in raw text ESI contexts
• make sure we're interpolating in the right contexts (i.e. inside a vars, when, try, long form assign, etc) and not others
• refactor EvalContext and Variables
• differentiate interpreter errors (they all come out as Parse errors right now)
• make the messages in those interp errors actually useful
• make sure errors in the interpreter all bubble out of Value::Error, rather than real errors
  • returning a Value::Error should log, but otherwise be interpreted as a Null (I think?!)
• fix all todo!() calls

[kailan]

Really pleased with this, thanks so much both for your hard work and for your patience while I get it reviewed. Just a few areas to address in regards to error handling but I'm happy for this to be released once the printlns are gone.

[kailan]

This is good to go! Thanks again. Really pleased about this.