Fix parsing errors on implementation keywords in conditional branches

[cirras]

In 5072681, we implemented keyword case-insensitivity directly into the grammar. This gave us a small performance boost, but quietly impaired our ability to track directiveNesting while lexing a source file.

We were using the case-sensitive String#startsWith to check for a particular compiler directive while lexing the token, which used to work because the input actually was lowercase before. Now, we need to do a case-insensitive comparison because the input is using the original casing.

This directiveNesting issue introduced some subtle issues in cases where:

• we're only parsing the interface of a file (skipping the implementation section
• implementation keywords are nested within conditional directives ($IF(DEF), $SLSE)
• the conditional directives are not lowercase

Fixes #299.

[fourls]

I'm always suspicious when I can't find something to nitpick about... but looks good to me!

[fourls]

Aha, found one! I think we need a changelog entry.

[fourls]

Looks good.