FTL: Add to_chars

Wrap std::to_chars for a slightly faster alternative to std::to_string
with optional radix.

The upcoming ftl/{enum.h,flags.h} imported from IF will use the binary/
hex options, while SF will stringify display IDs for trace labels using
the non-allocating version.

Bug: 185536303
Test: ftl_test
Change-Id: I587622763303d2baeb844e52eff55ce8f0bcbf1e