Full support for text collation

[sougou]

Vitess currently has limited support for handling collation. It's currently handled by outsourcing the work to mysql's weight_string function. But this approach works for only certain constructs.

There seems to be a bug in go's collation library: golang/go#23241. However, even if that bug is fixed, there is no guarantee that the library will exactly mimic mysql.

The longer term fix will likely involve building something from the ground up using the same unicode tables that mysql references. This will guarantee that vitess behavior matches mysql.

[harshit-gangal]

Closing in favor of #8606