ast: Change expression nodes to use l/r instead of children list
pp: Fix BOL detection to handle TK_AST's followed by TK_BEGIN/END
pp: Remove TK_EOL behind directives too
ppDirective(): Remove unnecessary if check
tk: Add tkSetAst()
tk: Fix tkToText() to reproduce the original token text properly
pp: Add expression evaluator
Add experimental #if block evaluator
parser: Update description
readme: Remove module descriptions, move into .bas files instead
readme: Update copyright and goal description