Refactoring: type description structures

Refactoring:

- Type description structures: new API based on structure inheritance
  rather than union,
- Rely on compound literals,
- Rename description fields: name -> event_name,
- Rename types (lttng_kernel_ namespace):
  - struct lttng_kernel_event_desc,
  - struct lttng_kernel_event_field,
  - struct lttng_kernel_ctx.
- Move some internal definitions (not used by probes) to
  events-internal.h.
- Streamline context registration API,
- Introduce the LTTNG_CREATE_FIELD_METADATA define for the tracepoint
  code generation pass which generates the type descriptions, allowing
  to describe structures as an array of fields, and use enumerations
  from those fields.
- Adapt all tracepoint instrumentation to those changes.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I9a968a62888c52438a1f62ec24fcced4c3fd6ddf