Macro generated class names are now forced to be valid.

This is a bit of a space bar heating restoration request, but I'll try my luck anyway ;)

Previously, I was using something like this for generic build:

```haxe
var name = 'some.package.Fancy<${param.toString()}>';//with @:genericBuild(...) class some.package.Fancy {}
var ret = macro class $name {
}
ret.meta.push({ name: ':native', params: [validClassName()], pos: pos });
```

I realize this stretches things a bit, but it was nice in that the type name shown to the user when e.g. accessing invalid fields would map very closely to what they had written, instead of some generated thing. Is there any chance we could revert to the previous behavior?