[MLton] Extra GC pressure

[Wesley W. Terpstra]

At the moment, the GC only cleans  up memory when the ML heap is full  
or explicitly requested. Memory usage from C data structures is not  
counted towards when the RAM is exhausted. How hard would it be to  
add MLton.GC.{add,sub}Memory that kept track of additional memory  
pressure in the GC?

The idea being that if you have an SML object that is backed by a C  
object (for example SQLite, GTK, etc), you can indicate to the GC how  
much memory (approximately) the C-side is consuming. When you free  
that memory, you tell the GC that the memory pressure has been  
reduced. The goal of this is to help trigger a GC earlier to clean up  
SML objects (and thus their attached C state).

A better interface would probably be MLton.GC.alloc <n> which returns  
an opaque type. When that type is GC'd, the <n> bytes are  
additionally subtracted from the extra memory pressure counter.

Good/bad idea? Easy/hard to implement idea?