Low-level generic signal call.

API: [opts] => depends on options [3]
Default  kind: none (optional)
Default level: none (must be provided)

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

If `:run` option is provided: returns value of given run form, or throws.
                   Otherwise: returns true iff call conditions were met.

Generic signals are fairly low-level and useful mostly for library authors or
advanced users writing their own wrapper macros. Regular users will typically
prefer one of the provided wrapper macros optimized for ease-of-use in
common cases.

These all use `signal!` underneath and offer the same options, but vary in
their defaults and the focus of their call APIs (args and return values):

  `event!` - (id      + opts/level) => true iff signal call was allowed
  `log!`   - (message + opts/level) => true iff signal call was allowed
  `error!` - (error   + opts/id)    => given error (unconditional)
  `trace!` - (form    + opts/id)    => form's result (value/throw) (unconditional)
  `spy!`   - (form    + opts/level) => form's result (value/throw) (unconditional)

Tips:

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

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