[Dependency Injection] Add macro based DI

[joshuawright11]

Leverage macros to simplify DI.

extension Container {
    @Singleton var database: Database = isTest ? .memory : .postgres(...)
}

// access database, ensuring it's resolved once
Container.$database

// override the value, i.e. in a test
Container.$database = .stub(...)

// `@propertyWrapper` sugar
@Injected(\.$database) var database

Will drop the $ syntax when (if) this issue is fixed.

Also in this PR:

• convert a decent chunk of tests to swift-testing
• remove AlchemyC in favor of Bcrypt implementation from hummingbird-auth
• restructure package structure