Users will want to set options from an environment variable or something
like that. Particular GC implementations will want to expose an
expanded set of options. For these reasons we make the options
interface a bit more generalized and include parsing.
The collector now has an abstract interface onto the embedder. The
embedder has to supply some functionality, such as tracing and
forwarding. This is a pretty big change in terms of lines but it's
supposed to have no functional or performance change.
We're targetting systems that need to be able to inspect the kind of an
object, so this information has to be somewhere. If it's out-of-line,
we might save memory, but we would lose locality. Concretely in Guile
the tag bits are in the object itself.
Also expand GC interface with "allocate_pointerless". Limit lazy
sweeping to the allocation size that is causing the sweep, without
adding to fragmentation.
