PublicKey constructor will quietly accept invalid inputs

[liquizard]

Status	Type	⚠️ Core Change
Ready	Bugfix	Yes

Problem

Encountered the error invalid transaction: Transaction failed to sanitize accounts offsets correctly when submiting a Serum SettleFunds command. Managed to pinpoint the root cause of the problem being a rouge space on the end of a public key string constant. Removing this space resolved the issue - this must cause some internal side-effect near the TransactionBuilder.

Further investigation found that PublicKey constructor would silently accept all manner of shenanigans.
Also found that PublicKey.IsValid implementation relies on Base58Encoder validation logic that would accept whitespace.

Solution

Added a FastCheck internal method to PublicKey and added calls to it from constructor and IsValid.
Added and modified a few tests.

Deploy Notes

This is potentially a breaking change to any client apps that accept public keys that include whitespace before or after and pass them directly to the constructor. On balance, I believe this enhancement will prevent more problems than it will cause.

[tiago18c]

Discuss comments. Otherwise looks good!

PS: I traced the actual problem to the method 'FindAccountIndex' in the 'MessageBuilder` class. I'll issue a fix later to this one! This fix should also address performance!
(root problem is that decoder was correctly trimming whitespace, but the message builder is inefficiently converting between binary and b58 multiples times and comparing the original pk string with a converted one that loses the trailing space. )

[tiago18c]

Thanks for the change. Don't merge yet, let me merge the other change and we can do 2PRs in 1 version