limit the number of VM regs to 16 (RV32e) #796

Job	Run time
CLI	4m 24s
Examples	7m 25s
Build binaries	3m 50s
Test (ubuntu-latest)	2m 28s
Test (runs-on, runner=8cpu-linux-arm64, run-id=12667306108)	1m 29s
Test (macos-latest)	1m 45s
Formatting & Clippy	25s
Test (Go) (ubuntu-latest)	1m 21s
Test (Go) (runs-on, runner=8cpu-linux-arm64, run-id=12667306108)	1m 6s
Test (Go) (macos-latest)	2m 9s
Test (Go) (windows-latest)	4m 42s
	31m 4s

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