Change the stash keybinding from 'z' to 'c'. And use 'i' for message ID.

In spite of being implemented with the word "stash" in the function
names, the documentation (and hence help strings) for each function
already use the word "Copy" to describe the action. So 'c' is a much
more natural key-binding, (particularly since 'z' didn't map to any
real word anyway).

We also use 'i' for the message ID copying of the submap. This is
intended to align mnemonically with the "id:" prefix already used
for message IDs.