Remove Sprache dependency

[jkoritzinsky]

Depends on #1667

• What does the pull request do?
  Change SelectorGrammar to use the Reader class used in binding path parsing (moved up to Avalonia.Markup in Allow LINQ Expressions for Binding Expressions #1667). Also, update Reader to be a ref struct with a ReadOnlySpan<char> to represent the member.

I've included some perf numbers below:

Sprache:
               Method |     Mean |    Error |   StdDev |   Gen 0 | Allocated |
--------------------- |---------:|---------:|---------:|--------:|----------:|
 ParseComplexSelector | 90.55 us | 1.764 us | 2.167 us | 52.2461 |  80.41 KB |

No Sprache:
               Method |     Mean |     Error |    StdDev |  Gen 0 | Allocated |
--------------------- |---------:|----------:|----------:|-------:|----------:|
 ParseComplexSelector | 3.921 us | 0.0695 us | 0.0879 us | 1.4420 |   2.22 KB |
Span-ified:
               Method |     Mean |     Error |    StdDev |  Gen 0 | Allocated |
--------------------- |---------:|----------:|----------:|-------:|----------:|
 ParseComplexSelector | 3.966 us | 0.0650 us | 0.0576 us | 1.1978 |   1.84 KB |

Sprache is the code in master. No Sprache is a Selector parser based on the Reader class that still uses strings and StringBuilders. Span-ified is the version at the tip of this PR branch (1133c3e).

Checklist:

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Avaloniaui.net with user documentation

[jkoritzinsky]

Looks like because of dotnet/android#1679 (comment) the Span-ified vesion is blocked on release of Visual Studio 15.8.

[jkoritzinsky]

There's a few workarounds in here for our Android projects that I can move into our packages so users don't need to work around it as well, but this PR is no longer blocked on Xamarin.Android!

@grokys @jmacato can you take a review pass at this?

[jmacato]

Looks great!

[grokys]

LGTM (Looks GREAT to me)!