0.1.1: improved error recovery

This is a small bug fixing release.

Let's start with parsing statistics:

Total parses: 3987;
successful parses: 3914;
failed parses: 73;
success percentage: 98.17%

Notable changes of this cycle:

Parsing improvements

• Added support trailing commas within a symbol declaration list (#39)

Implementation improvements

• Avoid emitting _layout_empty during error recovery (#40).
  This should provide a large improvement to the syntax tree in light of deeply-nested invalid code.