BuiltMessage and Kotlin extensions

[alexander-yevsyukov]

This PR introduces BuiltMessage interface which binds a builder type with the message type.

The binding is used in Kotlin extension declared (in the io/spine/base/extensions.kt file) as shown below:

inline fun <M: BuiltMessage<B, M>, B: ValidatingBuilder<M>>
        M.copy(block: B.() -> Unit): M {
    @Suppress("UNCHECKED_CAST") // ensured by the generated code
    val builder = this.toBuilder() as B
    block.invoke(builder)
    return builder.vBuild()
}

This binding is going to be used in other generated Kotlin extensions in the following PRs.

Changes to the existing types and hierarchies

• ValidatingBuilder interface moved to the io.spine.base package.
• RejectionType moved to the io.spine.base package (from io.spine.type).
• KnownMessage moved to the io.spine.type package (from io.spine.base).
• SerializableMessage moved to io.spine.type (from io.spine.base).
• Signal interfaces and EntityState are now are based on BuiltMessage and the code generation was adjusted accordingly.
• ThrowableMessage was renamed to RejectionThrowable to avoid the confusion. Now it's clear that a rejection has two incarnations: RejectionMessage and RejectionThrowable.
• UuidValue interface lost the unused generic parameter <I>.

Other notable changes introduced by this PR:

• smoke-tests were renamed to tests and pushed to the root project directory.
• MessageType.isSignal() method was introduced.
• Environment-related types moved to the package io.spine.environment.
• Interim artifacts named xxx-api were merged into corresponding xxx-base modules (e.g. spine-tools-api became a part of spine-tool-base).
• Some of the classes lost the Generated prefix (e.g. GeneratedNestedClass became NestedClass).
• Classes named XxxGenerator were renamed to be XxxGen.
• Data types that had incorrect validation options were removed. Please see this issue for details.

[alexander-yevsyukov]

I see the integration tests failing, I'm on the fix.

[armiol]

@alexander-yevsyukov LGTM in general. Please see my comments.

[dmdashenkov]

Please see my comments.

[armiol]

@alexander-yevsyukov LGTM with a single comment to address.