tracepoint: Refactor representation of nested types

tracepoint: Refactor representation of nested types

Refactor enumeration, sequence, array, structure, and variant types.

Implement internal data structures to support nested types.

All probe providers using ctf_enum(), ctf_array*() and ctf_sequence*()
are switched to this new binary layout, and the prior enum, array and
sequence abstract types are kept only for backward compatibility with
probe providers generated by older lttng-ust headers.

Each of sequence, array, struct and variant gain a "alignment" property,
which is a feature which was needed in lttng-modules to express
alignment for an array or sequence of bits.

The ust-ctl protocol with session daemon is extended to support those
new types.

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