runtime: implement precise GC

This implements the block-based GC as a partially precise GC. This means
that for most heap allocations it is known which words contain a pointer
and which don't. This should in theory make the GC faster (because it
can skip non-pointer object) and have fewer false positives in a GC
cycle. It does however use a bit more RAM to store the layout of each
object.

Right now this GC seems to be slower than the conservative GC, but
should be less likely to run out of memory as a result of false
positives.

compiler/llvm.go

@@ -84,6 +84,8 @@ func (c *compilerContext) makeGlobalArray(buf []byte, name string, elementType l
 // which words contain a pointer (indicated by setting the given bit to 1). For
 // arrays, only the element is stored. This works because the GC knows the
 // object size and can therefore know how this value is repeated in the object.
+//
+// For details on what's in this value, see src/runtime/gc_precise.go.
 func (c *compilerContext) createObjectLayout(t llvm.Type, pos token.Pos) llvm.Value {
 	// Use the element type for arrays. This works even for nested arrays.
 	for {