Co-authored-by: Matt Solomon <matt@mattsolomon.dev>

Standardizing on the term owner rather than signer except in and around calls to _getNSigners.

There was a bug in the remove all owners flow that would leave two owners on the safe.
Fixing that bug required using `swapOwners` in order to remove and
replace the last signer, since you cannot remove the only owner and then
add a new one (even when calling from a module).

Also added regression tests.

Also reorders the functions in the order they are called.

This required copy/pasting the SignatureDecoder code from the Safe repo.

This is more accurate because of the fact that showLiveness can be
used without approving a transaction.

h/t @mds1
Co-authored-by: Matt Solomon <matt@mattsolomon.dev>

OZ's EnumerableSet library is used to store the set of owners prior to
execution, and then to compare with the set of owners after execution,
and to add/remove those addresses from the mapping accordingly.

test(ctb): Add test for get75PercentThreshold

This change fixes a bug in getNSigners which incorrectly assumed that the length of the signature data
could be used to directly determine the number of signatures provided. This is wrong because contract signatures
append additional data used for the EIP1271 signature validation.

working for approvedHashes

this change is more readable