Create generic ftl serializer (#241)

* Start writing up the serializer

* Made sure indentation is handled properly

* Started handling select expressions

* Switched "    " to "\t" to make recognising indents easier

* Everything from TypeScript's "Serialize resource" test suite passes

* Added the rest of the tests

* Added a test to make sure subsequent entries with a comment aren't separated by newlines

* Fix serializer for fluent-syntax 0.11.0

Botch with only `&str` supported.

* Fix empty lines in messages being swallowed

* Fix indentation of default asterisk

It seems like `*` is supposed to go _inside_ the indent now.

* Fix lints

* Make `serialize()` generic over `Slice`

* Handle rare edge case of line terminating `\r`

* Fix redundant line break after after junk

* Align parsing of CRLF-terminated patterns

`\r\n` was parsed as a separate TextElement of `\n`, whereas `\n` is
parsed as part of the antecedent TextElement.

* Don't break line before leading dot pattern

* Don't implicitly trim when writing literals

* Replace roundtrip tests with manipulation tests

* Reintroduce roundtrip tests

* Move old fixtures into resource files.
* Test on unnormalized fixtures as well.

* Fix clippy lints

* Document serializer mod and pub functions

Also make Serializer private.

* Replace unwrap with expect

* Remove redundant error propogation

* BLACKLIST -> IGNORE_LIST

* Fix typo

* Mention serializer on changelog

* Revert c009674 and add crlf.ftl to ignore list

Co-authored-by: Michael Bryan <mbryan@autronics.com.au>

fluent-syntax/CHANGELOG.md

@@ -1,7 +1,7 @@
 # Changelog
 
 ## Unreleased
-
+  - Add module `serializer`.
   - …
 
 ## fluent-syntax 0.11.0 (February 9, 2021)

fluent-syntax/src/lib.rs

@@ -48,4 +48,5 @@
 //! ```
 pub mod ast;
 pub mod parser;
+pub mod serializer;
 pub mod unicode;