[SV] Introduce SystemVerilog `string` type

[mortbopet]

5616 introduces a new, high(er) level way of defining format string and printing in CIRCT.

Currently, this has a somewhat simple lowering that requires matching PrintOp(FormatVerilogStringOp(...)) into a sv.fwrite operation. The more correct way of doing this would be to have a separate lowering for each op, such that the FormatVerilogStringOp lowers to e.g. a $sformat(var, ...) which is then printed by e.g. $fwrite:

string v;
$sformat(v, ...)
$fwrite(fd, "%s", v)

however - this would require that said $sformat has a var to write to, which must have a sv string type. Currently, we have no way of describing nor emitting such an sv::InOutStringType which we'd presumably define by a sv::StringOp.

If the above were defined, we would probably also like to modify a bunch of operations in the SV dialect which currently takes a StringAttr but in theory would accept a sv::InOutStringType.

[teqdruid]

According to Bing's Chat (GPT), string format = $sformatf("%d %d %d", 1, 2, 3);.

[mortbopet]

Still, that doesn't seem like it can make us avoid having the string type, seeing as that format name still needs to be passed to $fwrite somehow.

[teqdruid]

Yeah, but for that we should just re-use hw.string. We use all of the other data types from hw in SV output and $sformatf doesn't require the string being passed in, so the model matches pretty perfectly. (Unless ChatGPT was wrong, though I'm pretty sure I checked.)