Removing sfYaml (unmaintained) in favor of Symfony\Yaml

[jaydiablo]

sfYaml has been abandoned (https://github.com/fabpot-graveyard/yaml) and I'd like to make sure that it works on new versions of PHP as they are released, unfortunately Doctrine's tests don't exactly cover a lot of the sfYaml code.

I considered forking it and getting tests running on travis, but that was proving to be more effort than it was worth (and we don't really want to have to maintain it if we don't have to). So I decided to try Symfony\Yaml in place of sfYaml, which seems to be a near drop-in replacement.

Nearly... this will be a major version bump of this lib, as Symfony\Yaml does throw some errors when the yaml contains invalid syntax (in our case this was duplicate keys, un-quoted strings that start with a percent sign and extra characters outside of quotes that shouldn't be there). IMO it's a good thing that the parser catches these, but any projects using this fork of Doctrine may need to fix their schema/fixture files to work with this parser.

FWIW, I did try optionally using the PECL Yaml extension if available, but that's strictly a Yaml 1.1 parser (sfYaml and Symfony\Yaml follow the 1.2 spec, specifically when it comes to converting values to boolean (i.e. unquoted Yes/No). The 1.2 parsers don't do this, but the 1.1 parsers do). I felt this was a big enough difference to not suggest/use the PECL Yaml parser here.

[codecov]

Codecov Report

Merging #43 into master will increase coverage by 0.5%.
The diff coverage is 40%.

@@             Coverage Diff             @@
##             master      #43     +/-   ##
===========================================
+ Coverage     69.66%   70.16%   +0.5%     
+ Complexity     7816     7607    -209     
===========================================
  Files           265      261      -4     
  Lines         19054    18561    -493     
===========================================
- Hits          13274    13024    -250     
+ Misses         5780     5537    -243

Impacted Files	Coverage Δ	Complexity Δ
lib/Doctrine/Core.php	60.81% <ø> (+0.36%)	97 <0> (-1)	⬇️
lib/Doctrine/Compiler.php	0% <0%> (ø)	14 <0> (-1)	⬇️
lib/Doctrine/Parser/Yml.php	53.84% <50%> (ø)	4 <0> (ø)	⬇️

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