Make channel.mark() throw and return offset()

[bradcray]

At present, the channel.mark() routine returns a syserr. It seems more natural for it to throw an error and to return channel.offset() for the callee to optionally use. With this capability, the new fast revcomp versions could be cleaned up modestly by combining the calls to offset() and mark() into a single call (without breaking backwards compatibility on the current version). While here, we should do the same for the _mark() version.

[bradcray]

@psahabu: If you're looking for some low-hanging error-handling fruit... @benharsh, @mppf: FYI.

Here's a start on it for the mark() version, though I didn't actually capture the error from qio_channel_mark() to throw it upward.

-inline proc channel.mark():syserr where this.locking == false {
-  return qio_channel_mark(false, _channel_internal);
+ inline proc channel.mark():int(64) throws where this.locking == false {
+  const offset = this.offset();
+  qio_channel_mark(false, _channel_internal);
+  return offset;
 }

[bradcray]

And here's the diff that I'd expect in the fast revcomps:

-      // capture the starting offset
-      const descOffset = input.offset();
-
       // Mark where we start scanning (keep bytes in I/O buffer in input)
-      input.mark();
+      const descOffset = input.mark();