VarChar and Char types lack ability to specify collation for parameters

[shueybubbles]

func (s *Stmt) makeParamExtra(val driver.Value) (res param, err error) {
switch val := val.(type) {
	case VarChar:
		res.ti.TypeId = typeBigVarChar
		res.buffer = []byte(val)
		res.ti.Size = len(res.buffer)
	case VarCharMax:
		res.ti.TypeId = typeBigVarChar
		res.buffer = []byte(val)
		res.ti.Size = 0 // currently zero forces varchar(max)

Key problems:

1. It's just sending the raw UTF8 bytes
2. If the column is encrypted, the insert will always fail due to a type mismatch.
3. The type aliases for string are too simplistic.

We can preserve backward compatibility of the aliases by allowing the app to specify a default collation for char and varchar types. We also need new types for per-parameter collation.