Add support for using keywords in type declarations

[nobodywasishere]

Closes #69, #39

[keidax]

I'm surprised at how much this balloons the parser size 😳
Any idea if that increase is spread evenly across the keywords, or if it's driven by a few in particular?

I'm looking into the new todo parse failures now.

[keidax]

The issue with

entries
  .select

is just a logic oversight in scan_whitespace. That should be a quick fix.

The issue with a keyword in a ternary seems more complicated. The Crystal parser has special handling to prevent type declarations inside a ternary, we may need to adopt some of that.

[nobodywasishere]

We can probably ignore the ternary issue for now, fixing this is more important. What do you think the fix is in scan_whitespace?