pgsql: Always use the caller-provided context for radix tree leaves

Always use the caller-provided context for radix tree leaves

Previously, it would not have worked for a caller to pass a slab
context, since it would have been used for other things which likely
had incompatible size. In an attempt to be helpful and avoid possible
space wastage due to aset's power-of-two rounding, RT_CREATE would
create an additional slab context if the value type was fixed-length
and larger than pointer size. The problem was, we have since added
the bump context type, and the generation context was a possibility as
well, so silently overriding the caller's choice may actually be worse.

Commit e8a6f1f908d arranged so that the caller-provided context is
used only for leaves, so it's safe for the caller to use slab here
if they wish. As demonstration, use slab in one of the radix tree
regression tests.

Reviewed by Masahiko Sawada

Discussion: https://postgr.es/m/CANWCAZZDCo4k5oURg_pPxM6+WZ1oiG=sqgjmQiELuyP0Vtrwig@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3e70da2781e86d07cd88f568ebeb38ddf7da8e8e

Modified Files
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src/include/lib/radixtree.h | 14 --------------
src/test/modules/test_radixtree/test_radixtree.c | 7 ++++---
2 files changed, 4 insertions(+), 17 deletions(-)