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>