Separate IDs of Wasm things from their names.

Previously, we eagerly generated string names for every Wasm thing,
and used that name both as their identity and as their debug name.

Now, we clearly separate those concepts: identities are instances
of abstract traits such as `FunctionID`, while debug names are
`OriginalName`s. Original names are only stored at the definition
site, while IDs are used to link definition and use sites.

IDs only need to have meaningful `equals` and `hashCode`. This
allows to define them as case classes that directly embed
`ClassName`s and `MethodName`s (among others), without having to
decode them or serialize them.

Moreover, since `OriginalName`s are internally represented as UTF-8
strings, we also avoid the cost of decoding and reencoding them
when writing binary `.wasm` files. The text writer gets more work
to do, but it is not on the performance-sensitive path, so this is
not really an issue.

For struct field IDs, this change also implies merging field *names*
and field *indices*. Since references to fields can now use
`FieldID`s as well, it removes a bit of bookkeeping at the
`ClassInfo` level. Both instance field indices and method table
entry indices are late-bound in the binary emitter. We do not have
to eagerly compute them.

wasm/src/main/scala/org/scalajs/linker/backend/WebAssemblyLinkerBackend.scala

@@ -77,7 +77,7 @@ final class WebAssemblyLinkerBackend(
 
     def maybeWriteWatFile(): Future[Unit] = {
       if (linkerConfig.prettyPrint) {
-        val textOutput = new TextWriter().write(wasmModule)
+        val textOutput = new TextWriter(wasmModule).write()
         val textOutputBytes = textOutput.getBytes(StandardCharsets.UTF_8)
         outputImpl.writeFull(watFileName, ByteBuffer.wrap(textOutputBytes))
       } else {