Refactoring: UST types public interfaces

Refactoring: UST types public interfaces

- Properly namespace public type interfaces with lttng_ust_ prefix,
- Move from union to parent embedding into children for inheritance.
- Introduce struct lttng_ust_type_common, which is fixed-size, to
  contain the type selector. Used as parent for each sub-type.
- Use compound literals on tracepoint probe definition rather than
  nesting into event field structure to allow each type to be extended
  with a struct_size scheme.
- The parent field is required to be the first field of each sub-type,
  allowing cast between parent and children types.
- Remove const-ness from various probe provider visible types to
  facilitate re-use of the code for eventual dynamically allocated
  structures.
- Remove "field_name" field from lttng_ust_ctx_field. Instead,
  dynamically allocate field names for each context.
- Introduce get_type_max_align() in lttng-context.c now used by
  lttng_context_update(). It performs a recursive traversal of the
  nested types rather than erroring out on recursive cases.
- Move "encoding" from integer type to array and sequences types.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: If99a2689f042b7e642b373e4646e9f85c009a47d