0.5.0: feature-complete

Once again, we start with parse statistics:

Total parses: 3987;
successful parses: 3950;
failed parses: 37;
success percentage: 99.07%

This is a big milestone, as right now, the only files that couldn't be
parsed are either invalid Nim (used to test the compiler) and/or Nim
using features that were part of one-off experiments that we decided
not to support.

This parser can now parses 100% of Nim.

Let's dive into the details

Parsing improvements

Grammar improvements

• Concept declaration without body is now supported.

• type(x) expressions are now supported. Note that while these
  generates (typeof) node, there are no plans to expand this special
  case beyond top-level in order to match how Nim handles this.

Layout improvements

• Indentation state of the current parse is now removed from the
  scanner. Instead the scanner will reconstruct this state from the
  token stream. This allowed us to remove the get_column dependency,
  speeding up the parser overall.