[class-parse] Use InvariantCulture to format floating-point values

[jonpryor]

Fixes: #129

The System.Double/Single "Round-trip Format Specifier" varies by
culture, so cultures which use , as the decimal separator character
use , within e.g. 42.0.ToString("r").

For example, the Java declaration:

class E {
	public static final double V = 42.5;
}

on french systems would result in class-parse emitting:

<field name="V" value="42,5" .../>

which, when run through generator, emits:

public const double V = (double) 42,5;

...which promptly fails with a CS1001. Oops.

Fix our double and float serialization in
XmlClassDeclarationBuilder so that CultureInfo.InvariantCulture is
used, which ensures that . is the decimal separator character,
ensuring that class-parse and generator output is as expected.