limit the number of VM regs to 16 (RV32e) #795

Job	Run time
CLI	4m 24s
Build binaries	3m 21s
Examples	7m 40s
Test (ubuntu-latest)	2m 42s
Test (runs-on, runner=8cpu-linux-arm64, run-id=12655157189)	1m 33s
Test (macos-latest)	1m 56s
Formatting & Clippy	23s
Test (Go) (ubuntu-latest)	1m 16s
Test (Go) (runs-on, runner=8cpu-linux-arm64, run-id=12655157189)	1m 10s
Test (Go) (macos-latest)	1m 54s
Test (Go) (windows-latest)	3m 37s
	29m 56s

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