Install RSpec
Add examples for how Proc#curry behaves
Replace real Proc#curry with a dummy implementation
Add simplest possible #curry implementation

This only works when the curried proc is called with all arguments, so
it’s not really currying it at all.
Return another proc when too few arguments are provided

This only works when the second call provides all remaining arguments.
Allow #curry to take an arity argument
Always curry the returned proc

This allows the arguments to be supplied in as many calls as necessary.
Support optional arguments
Check for too few arguments at currying time
Check for too many arguments at currying time
Indicate optional arguments in exception message