Verify and/or apply MDC extension X11/94-28: Portable String Length

Need to support at least 510 characters for node values. Initially, we would like to support at least 64KiB node values for locals, but preferably 1MiB or more. For globals, we need to meaningfully support block sizes other than 1KiB to get larger node values, and eventually will need to have nodes that span multiple blocks. Though having a disparity between the length limits in locals and globals is suboptimal, it will likely be necessary for some time.