feat: add realm /r/demo/math_eval #1450

Job	Run time
build (1.20.x, GOARCH=amd64 GOOS=linux, gno)	39s
build (1.20.x, GOARCH=wasm GOOS=js, gno)	41s
build (1.21.x, GOARCH=amd64 GOOS=linux, gno)	44s
build (1.21.x, GOARCH=wasm GOOS=js, gno)	33s
test (1.20.x, _test.cmd)	1m 50s
test (1.20.x, _test.pkg)	1m 8s
test (1.20.x, _test.gnolang.native)	56s
test (1.20.x, _test.gnolang.stdlibs)	1m 24s
test (1.20.x, _test.gnolang.realm)	1m 1s
test (1.20.x, _test.gnolang.pkg0)	2m 29s
test (1.20.x, _test.gnolang.pkg1)	8m 35s
test (1.20.x, _test.gnolang.pkg2)	5m 44s
test (1.20.x, _test.gnolang.other)	56s
test (1.21.x, _test.cmd)	1m 42s
test (1.21.x, _test.pkg)	1m 12s
test (1.21.x, _test.gnolang.native)	1m 1s
test (1.21.x, _test.gnolang.stdlibs)	1m 54s
test (1.21.x, _test.gnolang.realm)	1m 4s
test (1.21.x, _test.gnolang.pkg0)	3m 6s
test (1.21.x, _test.gnolang.pkg1)	7m 52s
test (1.21.x, _test.gnolang.pkg2)	5m 47s
test (1.21.x, _test.gnolang.other)	55s
	51m 13s

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