Make the "environment" parameter to `@JavaMethod` initializers optionally-typed and default to `nil`

[DougGregor]

Right now, an initializer of an @JavaClass type requires an environment parameter, e.g.,

@JavaClass("org.swift.javakit.examples.MyClass")
struct MyClass {
  @JavaMethod
  init(environment: JNIEnvironment)
}

We could allow this to be:

@JavaClass("org.swift.javakit.examples.MyClass")
struct MyClass {
  @JavaMethod
  init(environment: JNIEnvironment? = nil)
}

And, if nil is passed, use JavaVirtualMachine.shared().environment() (introduced in #69) as the default value. This requires some work on the @JavaMethod macro implementation, and collapsing JavaKitVM down into JavaKit, but should otherwise be straightforward.