Change VT Parser to use char instead char32_t

[christianparpart]

Use char instead of char32_t as input.

Later then in Ground state and OSC (/DCS) should decode UTF-8 into UTF-32 payloads when UTF-8 is to be expected.

That should definitely improve performance. But how much?

When doing this, the new Cell struct could just store a char instead of char32_t and filor any higher codepoints defer to CellExtra. saves 3 bytes for the standard case.

[christianparpart]

merged