"Log" signal call, focused on message + level.

API: [msg] [level-or-opts msg] => true iff signal call was allowed.
Default  kind: `:log`
Default level: `:info`

When conditions are met [1], creates a Telemere signal [2] and dispatches it to
registered handlers for processing (writing to console/disk/db, etc.).

Examples:

  (log! "My msg")       ; %> {:kind :log, :level :info, :id ::my-id ...}
  (log! :warn "My msg") ; %> {...         :level :warn ...}
  (log!
    {:let  [x "x"] ; Available to `:data` and `:msg`
     :data {:x x}}

    ["My msg:" x]) ; %> {... :data {x "x"}, :msg_ "My msg: x" ...}

Tips:

  - Test using `with-signal`: (with-signal (log! ...)).
  - Supports the same options as other signals [3].

  - Prefer `event!` to `log!` by default, since it better encourages structured
    data over unstructured messages.

  - `msg` arg may be a string, or vector of strings to join with `\space`.
  - See also `msg-splice`, `msg-skip` utils.

----------------------------------------
[1] See `help:signal-flow`     docstring
[2] See `help:signal-content`  docstring
[3] See `help:signal-options`  docstring