fix(encoder,parser): escape and unescape strings

[goto-bus-stop]

This PR improves the string value printing in apollo-encoder. It tries to sensibly decide between a multi-line block string, a single-line block string, and a non-block string depending on the contents it needs to print. It escapes special characters when writing non-block strings. When writing block strings, only """ is escaped as \""" so the block isn't ended prematurely.

The parser's String::from(StringValue) implementation also un-escapes characters.

[goto-bus-stop]

I'm not really happy with this but I think I should revisit later rather than spend more time on it now. I'd like to refactor string lexing (to validate escapes in more detail) and parsing and also address #319 and block string indentation at that time. For now this PR should improve the string printing in apollo-encoder so it doesn't produce invalid output anymore.

[lrlna]

a comment on formatting block string values. otherwise looks good!

yea, string parsing needs a brand new coat of paint. will probably be easier to solve multi-error lexing issue too.

[goto-bus-stop]

the parser unescaping implementation that i have here does the wrong thing for block strings btw. As block strings don't actually support escapes except for \"""", but they will be processed by this PR. I'd improve that in the future string improvements.