Improve startup of memory access var handles by simplifying combinator chains

The code that generated memory access var handles given a call to `MemoryLayout::varHandle(PathElement...)` is very general. As such, while it works on any possible selected chain of path elements, it's also poorly optimized for the common cases (e.g. simple struct field access). In such common cases, the runtime generates too many lambda forms, which end up degrading startup.