Call a Telemere signal creator to conditionally create a signal at that callsite.

When filtering conditions are met [4], the call creates a Telemere signal [3]
and dispatches it to registered handlers for processing (e.g. writing to
console/file/queue/db, etc.).

Telemere doesn't make a hard distinction between different kinds of signals
(log, event, error, etc.) - they're all just plain Clojure/Script maps with
various keys:

  - All signal creators offer the same options [2], and
  - All signal kinds can contain the same content [3]

Creators vary only in in their default options and call APIs (expected args
and return values), making them more/less convenient for certain use cases:

  `log!` ---------- [message + opts/level] => true iff signal was created (allowed)
  `event!` -------- [id      + opts/level] => true iff signal was created (allowed)
  `error!` -------- [error   + opts/id   ] => given error (unconditional)
  `trace!` -------- [form    + opts/id   ] => form result (value/throw) (unconditional)
  `spy!` ---------- [form    + opts/level] => form result (value/throw) (unconditional)
  `catch->error!` - [error   + opts/id   ] => form value, or given fallback
  `signal!` ------- [          opts      ] => depends on options

- `log!` and `event!` are both good default/general-purpose signal creators.
- `log!` emphasizes messages, while `event!` emphasizes ids.
- `signal!` is the generic creator, and is used by all the others.

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[2] See `help:signal-options` - {:keys [kind level id data ...]}
[3] See `help:signal-content` - {:keys [kind level id data ...]}
[4] See `help:signal-flow`    - (filters, handling, etc.)