Custom parser (without Flex+Bison)

Flex and Bison were a good starting point for the compiler, and a good learning experience, but the architecture is being hindered by them.

Reasons:

• Flex and Bison are not particularly good at multithreading
• More helpful error messages to the parser
  • e.g. for the parentheses requirement in some boolean operations - difficult with Bison
• Less dependencies for the compiler

I want to rewrite the lexer+parser into completely custom code.