Add repl support (including live coding with Emacs via geiser) #19

Job	Run time
build (linux, single, x86_64, template_debug, ubuntu-20.04)	9m 13s
build (macos, single, universal, template_debug, macos-latest)	9m 12s
build (android, single, arm64, template_debug, ubuntu-20.04)	7m 40s
build (android, single, arm32, template_debug, ubuntu-20.04)	6m 35s
build (android, single, x86_64, template_debug, ubuntu-20.04)	7m 38s
build (android, single, x86_32, template_debug, ubuntu-20.04)	6m 38s
build (ios, single, arm64, template_debug, macos-latest)	4m 13s
build (web, single, wasm32, template_debug, ubuntu-20.04)	11m 5s
build (linux, single, x86_64, template_release, ubuntu-20.04)	8m 21s
build (linux, double, x86_64, template_debug, ubuntu-20.04)	8m 17s
build (macos, double, universal, template_debug, macos-latest)	6m 57s
build (android, double, arm64, template_debug, ubuntu-20.04)	7m 24s
build (android, double, arm32, template_debug, ubuntu-20.04)	6m 33s
build (android, double, x86_64, template_debug, ubuntu-20.04)	7m 51s
build (android, double, x86_32, template_debug, ubuntu-20.04)	6m 45s
build (ios, double, arm64, template_debug, macos-latest)	4m 54s
build (web, double, wasm32, template_debug, ubuntu-20.04)	11m 18s
build (linux, double, x86_64, template_release, ubuntu-20.04)	8m 30s
merge	5s
	2h 19m 9s

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