InvariantCulture for operand to string conversion in Instruction.ToString()

[Fantoom]

Instruction.ToString() uses the culture of the current thread, which can be problematic for converting doubles and floats to a string.

[ForNeVeR]

For context: we're writing a C compiler using Cecil, and hit the issue while dumping the methods' IL code and comparing it in the compiler unit tests (one contributor had a locale that uses different number format, and broke the tests). So, I agree that these changes should be made.

[jbevain]

Looking good, thank you!