Allow ulong range values to coerce to long bits

[headius]

This adds ulong versions of the numeric-to-long conversion methods
that allow ulong-ranged values to be coerced to a Java long.
Callers must take care to use the resulting long correctly, since
Java will always see it as signed.

Added primarily to support ulong use cases in FFI, and fixes #6376
and a test commented out due to jnr/jffi#87.

The tests in question can be enabled via the following patch:

diff --git a/spec/ffi/struct_spec.rb b/spec/ffi/struct_spec.rb
index 70e60c15a9..b5e262a04a 100644
--- a/spec/ffi/struct_spec.rb
+++ b/spec/ffi/struct_spec.rb
@@ -365,17 +365,17 @@ module StructSpecsStructTests
     int_field_test(:int, [ 0, 0x7fffffff, -0x80000000, -1 ])
     int_field_test(:uint, [ 0, 0x7fffffff, 0x80000000, 0xffffffff ])
     int_field_test(:long_long, [ 0, 0x7fffffffffffffff, -0x8000000000000000, -1 ])
-    if RUBY_ENGINE != 'jruby' # https://github.com/jnr/jffi/issues/87
+    # if RUBY_ENGINE != 'jruby' # https://github.com/jnr/jffi/issues/87
       int_field_test(:ulong_long, [ 0, 0x7fffffffffffffff, 0x8000000000000000, 0xffffffffffffffff ])
-    end
+    # end
     if FFI::Platform::LONG_SIZE == 32
       int_field_test(:long, [ 0, 0x7fffffff, -0x80000000, -1 ])
       int_field_test(:ulong, [ 0, 0x7fffffff, 0x80000000, 0xffffffff ])
     else
       int_field_test(:long, [ 0, 0x7fffffffffffffff, -0x8000000000000000, -1 ])
-      if RUBY_ENGINE != 'jruby' # https://github.com/jruby/jruby/issues/6376
+      # if RUBY_ENGINE != 'jruby' # https://github.com/jruby/jruby/issues/6376
         int_field_test(:ulong, [ 0, 0x7fffffffffffffff, 0x8000000000000000, 0xffffffffffffffff ])
-      end
+      # end
     end
 
     it ":float field r/w" do

[ahorek]

thanks for looking into this! 👍