Cleanup utf in module

[benjones]

Eliminate gotos in the UTF reencoding code (taking UTF16/32 files and encoding them as UTF 8). It also replaces pointer + length with slices during processing.

I think the new function I made should probably get pulled out and put with other UTF code, perhaps DRuntime?

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[WalterBright]

see my comments

[thewilsonator]

@WalterBright any more comments?

[thewilsonator]

@WalterBright all good now?

[thewilsonator]

Thanks!