[NFC] [Serializer] Pack information in serializer (#69287)

Previously, the boolean values will occupy spaces that can contain
integers. It wastes the spaces especially if the boolean values are
serialized consecutively. The patch tries to pack such consecutive
boolean values (and enum values) so that we can save more spaces and so
the times.

Before the patch, we need 4.478s (in my machine) to build the std module
(https://libcxx.llvm.org/Modules.html) with 28712 bytes for size of the
BMI. After the patch, the time becomes to 4.374s and the size becomes to
27388 bytes for the size of the BMI.

This is intended to be a NFC patch.

This patch doesn't optimize all such cases. We can do it later after we
have consensus on this.