We can have a 0 byte in the *middle* of an encoding.

We can't allow NULL with the new variant (needed by talloc's set_destructor
for example), so document that and expose all three variants for different
uses.

cast_if_type() should not try to degrade the expression using 1?(test):0,
as that promotes bool to int, as well as degrading functions to function
pointers: it should be done by the callers.

In particular, this fixes sparse_bsearch.

This means we can't have more than 2^25 elements per page; that's
a maximum small page size of about 2^24 (with >8 objects per small page
we move to large pages), meaning a poolsize max of 4G.

We have a tighter limit at the moment anyway, but we should remove it
once we fix this.  In particular count all-zero and all-one words in
the used field (that's what we care about: full or empty) would give us
another factor of 64 (we only care about larger pool sizes on 64-bit
platforms).

We can also restore the larger number of pages and greater inter-page
spacing once we implement the alternative tiny allocator.

Turns out that now we use page numbers, this is more fundamental.

This version has limitations: pools must be at least 1MB, and allocations
are restricted to 1/1024 of the total pool size.

Particularly useful for building tests standalone.

To do this, we have to lose the ability for preargs and postargs to allow const and volatile argument signatures.

hashtable: make traverse callback typesafe.

This takes my "make fastcheck" from about 57 seconds to about 43 seconds.

The ccanlint patch is rather intrusive.  First, it adds a new field to
all the ccanlint tests, "key".  key is a shorter, still unique
description of the test (e.g. "valgrind").  The names I chose as keys
for all the tests are somewhat arbitrary and often don't reflect the
name of the .c source file (because some of those names are just too
darn long).  Second, it adds two new options to ccanlint:

   -l: list tests ccanlint performs
   -x: exclude tests (e.g. -x trailing_whitespace,valgrind)

It also adds a consistency check making sure all tests have unique
keys and names.

The primary goal of the ccanlint patch was so I could exclude the
valgrind test, which takes a really long time for some modules (I
think btree takes the longest, at around 2 minutes).  I'm not sure I
did it 100% correctly, so you'll want to review it first.