This eliminates 1945 (432709 -> 430764) allocations from running
`./tests/SelfTest -o /dev/null`. In general terms, this saves
an allocation every time an unvisited `SECTION` is passed, which
means that the saved allocations are quadratic in number of sibling
(same level) `SECTION`s in a test case.

Now we delay allocating owning `NameAndLocation` instances until
we construct a new tracker (because a tracker's lifetime can be
significantly different from the underlying tracked-thing's name).

This saves 4239 allocations (436948 -> 432709) when running
`./tests/SelfTest -o /dev/null`, at some cost to code clarity
due to introducing a new ref type, `NameAndLocationRef`.

This removes 109 allocations from running `tests/SelfTest`
(437057 -> 436948).

This always saves 1 allocation per benchmark, and another two
allocations if the benchmark name is longer than the SSO buffer.

When running `./tests/SelfTest -o /dev/null`, this saves 109
allocations (437167 -> 437058).

When running `./tests/SelfTest -o /dev/null`, this saves 1272
allocations (437439 -> 437167). In general, this saves multiple
allocations per end of an entered `SECTION`, if the section name
was too long for SSO, because `RunContext::sectionEnded` can then
move the section's name further down the callstack.

When running `./tests/SelfTest -o /dev/null`, this saves 468
allocations (438907 -> 437439). In general, this saves 1 allocation
every time an entered `SECTION` ends and the section name was long
enough to move out of the SSO buffer.