Avoid charset lookup machinery in java.nio.charset.StandardCharsets

The public fields of StandardCharsets are part of the public API, so they definitely need to exist. So let's create them directly (i.e. using "new", not Charset.forName) and don't bother putting these fields anywhere else (i.e. get rid of the INSTANCE singleton fields)

(Motivation: I ran into trouble using reflection via StandardCharsets.<clinit> too early during bootstrap with a local mod.)