Replace assert() -> BT_ASSERT() and some preconditions with BT_ASSERT_PRE()

In the whole project: replace assert() with BT_ASSERT(), and include
<babeltrace/assert-internal.h> instead of <assert.h>. This means that,
from now on, logic/postcondition assertions are disabled by default: you
need to enable them explicitly with `BABELTRACE_DEBUG_MODE=1` when
configuring the project.

In the library: replace the precondition checks which actively occur
while a graph is running with BT_ASSERT_PRE() or other helpers that are
only enabled in developer mode. Those are mostly checks for `bt_event`,
`bt_packet`, `bt_field`, `bt_notification`, and `bt_value` objects.

Frozen and validation flags for those objects are now only handled in
developer mode. The concept of set/unset fields is also only handled in
developer mode. This is why some internal functions have their name
prefixed with `_` and a macro is used instead to do nothing when in
non-developer mode (e.g., bt_field_freeze() calls _bt_field_freeze() in
developer mode and does nothing in non-developer mode).

The transformation of unconditional run-time precondition checks to
using BT_ASSERT_PRE() is not done for the notification iterator object.
This is a separate concern with different challenges which will be
implemented in a subsequent patch.

Tests which check that API functions fail when preconditions are not
satisfied in `tests/lib/test_bt_values.c` and
`tests/lib/test_ctf_writer.c` are removed. From my point of view,
checking for those preconditions is not part of the API now: it is only
a service provided for developers in developer mode. In other words,
because a library user cannot expect a specific reaction when not
satisfying the required preconditions, you cannot unit-test them either.
What is important is to validate that the library works as expected when
used as expected.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>