Add a new 'path' option to the REPL

[robdockins]

This lets the user reset the module search path. It is a bit of a
blunt instrument as it resets the entire path. Perhaps we should also
add the ability to add new paths to the front of the search order,
e.g. with a new :prependpath command?

Fixes #631

[pnwamk]

Semi-related (?) -- we recently merged a PR that gives remote clients the ability to "extend" the search path with new paths (while retaining the old and ensuring there are no duplicates).

PR: #1137

Relevant code:

cryptol/cryptol-remote-api/src/CryptolServer.hs

Lines 147 to 149 in c2ef506

	extendSearchPath :: [FilePath] -> ServerState -> ServerState
	extendSearchPath paths =
	over moduleEnv $ \me -> me { meSearchPath = nubOrd $ paths ++ meSearchPath me }

I'm not sure there needs be a direct relationship between this (or future) addition(s) to the REPL per se, but just in case it's worth considering this broader feature on all fronts, there you go.