Prevent exception caused by mounted, module-contained Crepe APIs

Object.const_set doesn't work with invalid constant names, so let's make
sure we gsub-em out.

Fixes #69.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Simplify code

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Streaming tweaks: yield stream to block, privatize ivar

This should avoid collisions with any @stream variable that people may
use, and it should make it more intuitive to write directly to the
stream in the streaming block.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>