Started implementation of comparator execution support.

src/lib-sieve/ext-envelope.c

src/lib-sieve/ext-fileinto.c

src/lib-sieve/ext-reject.c

src/lib-sieve/plugins/vacation/ext-vacation.c

src/lib-sieve/sieve-binary.c

src/lib-sieve/sieve-binary.h

src/lib-sieve/sieve-code.c

src/lib-sieve/sieve-comparators.c

src/lib-sieve/sieve-comparators.h

src/lib-sieve/sieve-extensions.h

src/lib-sieve/sieve-interpreter.c

src/lib-sieve/sieve-interpreter.h