Switch to sqlite3's lemon parser generator.

This commit drops the flex + bison code in favor to a hand-written
lexer and a new grammar file in lemon syntax.

The change results in a much smaller binary and easier to maintain
code. Code required to build and maintain the AST has been split
off into the ast.c and ast.h files.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>