Replace repetitive C code in "gc.c" and "vfasl.c" with an
implementation using a little "Parenthe-C" language, which is a
somewhat declarative description of object tracing. From that
descrition, we generate different kinds of tracing functions, such as
the copy function or the sweep function.
The little language is still bascially C, just with parentheses and
parameterization that is much better than trying to use the C
preprocessor. (The "mkgc.ss" file includes the compiler from
Parenthe-C to C.)
Besides replacing existing code, we also generate a new traversal to
implement `compute-object-sizes`. Finally, the GC can now perform a
fused `collect` and `compute-object-sizes` in a single traversal.
Also improve the way that locked objects are detected during GC. This
can make a significant difference (on the order of 10-20% for a full
collection) when locked objects are long-lived.
original commit: de1f5c41d729ac75822a1f1e633ec6d042c883dc
