Conversion functions that consume should be called into_*, not to_*

Offenders:

% git grep 'fn to_.*(self'|cat
openpgp/src/autocrypt.rs:    pub fn to_tsk(self) -> TSK {
openpgp/src/message/mod.rs:    pub fn to_packet_pile(self) -> PacketPile {
openpgp/src/packet/aed.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/compressed_data.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/key.rs:    pub fn to_packet(self, tag: Tag) -> Result<Packet> {
openpgp/src/packet/literal.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/mdc.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/one_pass_sig.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/pkesk.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/seip.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/signature/mod.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/skesk.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/skesk.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/unknown.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/user_attribute.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet/userid.rs:    pub fn to_packet(self) -> Packet {
openpgp/src/packet_pile.rs:    pub fn to_packet_pile(self) -> Result<PacketPile> {
openpgp/src/parse/packet_pile_parser.rs:    pub fn to_packet_pile_parser(self) -> Result<PacketPileParser<'a>>
openpgp/src/tpk/mod.rs:    pub fn to_packets(self) -> Vec<Packet> {
openpgp/src/tpk/mod.rs:    pub fn to_packet_pile(self) -> PacketPile {