Remove old method names from Rune

[GrabYourPitchforks]

The following methods should be removed from the Rune sources since they were renamed in a recent commit. (The old names were left in the sources temporarily to avoid build breaks.) Left side is the old name, right side is the new name.

TryEncode -> TryEncodeToUtf16
TryEncodeAsUtf8Bytes -> TryEncodeToUtf8
DecodeUtf16 -> DecodeFromUtf16
DecodeUtf16FromEnd -> DecodeLastFromUtf16
DecodeUtf8 -> DecodeFromUtf8
DecodeUtf8FromEnd -> DecodeLastFromUtf8

At the same time we should re-enable the test suppressed below.

https://github.com/dotnet/coreclr/blob/31581af5fa816fb2ea94145823ec3bdd6c0b0327/tests/CoreFX/CoreFX.issues.json#L1257

At the same time we should also fix up the parameter names on the Utf8 static class. The parameter names in the reference assembly in corefx are correct; the parameter names in the actual implementation in coreclr are incorrect.

[GrabYourPitchforks]

Changes to the Utf8 static class were addressed in dotnet/coreclr@effeb6e.