Build with Idris2 0.5.1

[mixphix]

There's only two things stopping master from compiling with 0.5.1, the last released version of the language; one which a package manager for that language should generally support.

• pclose doesn't return an error code yet, so always return 0 (bad, probably, but at least it builds with the version the rest of the ecosystem is using at the moment!) there's probably a better fix that's not too difficult
• system doesn't unwords yet, so manually do that