Support for SQLCipher 4

[groue]

This PR makes GRDB able to link with both SQLCipher 3 and 4, and will ship with GRDB 4 (#479).

• The only supported way to use GRDB with SQLCipher is through CocoaPods. Other installation methods are no longer documented or supported.
• The GRDBCipher CocoaPod is no longer, replaced with the GRDB.swift/SQLCipher subspec.

In a totally unrelated fashion, this PR also removes all former attempts at supporting Carthage (a long series of repeated failures and lack of commitment from everybody). Let's cut this dead branch.

• Run and test GRDB with SQLCipher 4;
• Test GRDB.swift/SQLCipher CocoaPod subspec.
• Update documentation about installation of GRDB with SQLCipher support.
• Write documentation about the transition to SQLCipher 4 (@Marus, @michaelkirk-signal, are you still interested in writing such a guide? Your help would be welcome).