refactor: move mock code into new mock mod

[dan-da]

This moves all "extra" functionality into a new module "mock" with a corresponding feature flag and its own Error enum.

The moved items are:

 GenesisBuilderMock --> mock::GenesisBuilder
 SpentbookNodeMock  --> mock::SpentbookNode
 SimpleKeyManager   --> mock::KeyManager
 SimpleSigner       --> mock::Signer

This makes the core library leaner, meaner and easier to audit/review/understand.

It also makes it extra clear what is mock functionality when using the lib. eg, in test cases, mint-repl or sn_dbc_examples.

Cargo changes:

• add 'mock' feature flag
• set mock feature flag when building tests

Code changes:

• use mock::* in tests, reissue, mint-repl
• move GenesisBuilderMock into mock::GenesisBuilder
• remove crate::Error::Spentbook*
• add crate::Error::Mock, only when mock feature enabled.
• move SimpleKeyManager, SimpleSigner into mock::{KeyManager, Signer}
• expose pub crate::mock in lib.rs when feature enabled
• move SpentbookNodeMock into mock::SpentbookNode
• add mock::Error

[dan-da]

note: cargo tests are failing. Once #168 is merged, I will rebase this on top.

[dan-da]

rebased on latest master. I will merge it once CI tests pass.