pw_log: Reasonable defaults for level/flag bits
pw_log: Reasonable defaults for level/flag bits

This sets reasonable, but restricted, bit limits for the number of bits
available to store log flags and level. This is necessary for cases like
tokenized logs and asserts where log call site size matters; packing
levels and flags into a single word can make large savings in binary
size.

Change-Id: I6a2f1c58c3000e64a361d8430d0e82b75bd0bcf0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/16180
Commit-Queue: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

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