This is a huge patch that should be a global NOP.
For unix type platforms it enables -Wconversion to issue warnings (-> error)
for all automatic casts that seem less than ideal but are normally concealed
by the toolchain.
This is things like passing an int to a size_t argument. Once enabled, I
went through all args on my default build (which build most things) and
tried to make the removed default cast explicit.
With that approach it neither change nor bloat the code, since it compiles
to whatever it was doing before, just with the casts made explicit... in a
few cases I changed some length args from int to size_t but largely left
the causes alone.
From now on, new code that is relying on less than ideal casting
will complain and nudge me to improve it by warnings.
Optimizations for memory-tight systems.
Check all previous gaps first for any usage, so gaps we created when
faced with perhaps a relatively large allocation that left a lot of
the last chunk on the table can be backfilled with smaller things as
it goes on.
Separate the members that only live in the head object out of the
buffer management object, reducing the cost of new chunks. Allocate
the head object members as the first thing in the first chunk, and
adjust all the code to look there for them.
