Defines a component with keyword k and optional metadata attribute-map followed by system implementations (via defmethods).

attr-map may contain :let binding which is let over the value part of a component [k value].

Example:

(defsystem foo)

(defc :foo/bar
  {:let {:keys [a b]}}
  (foo [_]
    (+ a b)))

(foo [:foo/bar {:a 1 :b 2}])
=> 3

Defines a component without systems methods, so only to set metadata.

A system is a multimethod which takes as first argument a component and dispatches on k.

Map of all systems as key of name-string to var.