Merge pull request #256 from david-christiansen/providers

Dynamic C libs, interpreter IO, :x command, start of type providers
Merge pull request #257 from david-christiansen/providers

Make dynamic library extension platform-dependent
reduce duplication by providing a show instance for CmdArg
Merge pull request #258 from hannesm/reduce-code-duplication

reduce duplication by providing a show instance for CmdArg
Tweak linearity checker to deal with overloaded names properly
Minor effects changes (locally, search depth)
Remove type class argument restriction in parser
add command to change current directory
Merge pull request #260 from hannesm/support-change-of-working-directory

add command to change current directory