Description:
This PR provides a new annotation InlineSource(language: String, source: String) which allows to embed source snippets into the NIR generated for a class or object, e.g.

@InlineSource("C","int foo(int i) { /* ... */ }")
@extern
object Foo {
  def foo(i: Int): Int = extern
}

These source snippets can then be processed with sbt plugins by registering an InlineSourceHook to the task key nativeInlineSourceHooks.

Use Cases:

• embed small external helper functions
• on-the-fly generation of wrapper code (e.g. for C++ libraries) with macros (i.e. the macro generates the InlineSource snippet that contains the C wrapper around some C++ type).

Changes:

• add new annotation InlineSource(source,language)
• add new NIR tag InlineSrcAttr
• handle tag in BinarySerializer/BinaryDeserializer
• collect all InlineSource attrs during linking
• add Config.inlineSourceHooks; registered hooks are called with the list of InlineSource attrs after optimization is completed => only InlineSource fragments that are attached to classes/objects which are actually linked into the binary will be passed to the hooks
• provide sbt settingKey nativeInlineSourceHooks to register hooks that should be called after optimization is completed