Cleaned up ast implementation a little.

src/lib-sieve/plugins/include/ext-include-common.c

src/lib-sieve/plugins/include/include.sieve

src/lib-sieve/plugins/include/included2.sieve

src/lib-sieve/sieve-ast.c

src/lib-sieve/sieve-ast.h

src/lib-sieve/sieve-parser.c

src/lib-sieve/sieve.c