Add Format()

[mcdee]

This adds a Format() function to decimal, allowing for setting of a thousands separator (as requested in #267) and decimal separator, as well as the existing functionality for trimming trailing zeros. This allows for easier use in localized environments.

This replaces the prior internal string() function, which could now be removed entirely but that would result in a larger PR so has been left for now.

Worth taking a close look at lines 1032-1034 to ensure they behave as expected, as they alter the flow compared to before this PR.