Check file-private types for abstract defs and recursive structs

[HertzDevil]

Unimplemented abstract defs of file-private types do not trigger a compilation error until they are called:

private abstract class Foo
  abstract def foo
end

class Bar < Foo
  # should not compile
end

Recursive value types do not trigger compilation errors at all:

private struct Foo
  def initialize(@x : Foo?)
  end
end

Foo.new(Foo.new(nil)) # Invalid memory access (signal 11) at address 0x0

This happens because both Crystal::AbstractDefChecker and Crystal::RecursiveStructChecker only look at types reachable from the top-level namespace (i.e. the Crystal::Program instance); this PR adds checks for the namespaces associated with each file-private scope.

[asterite]

Brilliant!