Rename `AsBytes` to `IntoBytes`

[joshlf]

Progress

• Decide between ToBytes and IntoBytes
  • Decided to go with IntoBytes so that FromBytes/IntoBytes is symmetrical with stdlib's From/Into
• Implementation
  • In progress in #700

Description

Now that AsBytes permits UnsafeCells as part of #251, it can be implemented for types which support value-only transmutation (ie, for which fn as_bytes(&self) -> &[u8] where Self: NoCell is not available because Self: !NoCell). Given this change, a more appropriate name for the trait would be IntoBytes. This also brings our pair of FromBytes/IntoBytes into line with the existing pattern of From/Into.