Envelope: added compile-time envelope-part verification.

TODO

doc/rfc/RFC Controversy.txt

sieve/errors/envelope-errors.sieve

sieve/examples/elvey.sieve

src/lib-sieve/ext-envelope.c

src/lib-sieve/sieve-ast.c

src/lib-sieve/sieve-ast.h

src/lib-sieve/tst-address.c