v0.3.0

Added

• Add sn_std_allocator, the standard library backed allocator, moved here
  from SnCore where it did not belong
• Add std_allocator.h to the snmemory.h umbrella header
• Build the tests against a shared library in CI, which is what catches a
  symbol that is missing an export macro

Changed

• Build against SnCore v0.3.1, which fixed the variable length integer
  helpers. The freelist stores the distance to its node header in one of them,
  so that fix corrects the freelist too

Known issues

• sn_std_allocator realloc does not honour a changed alignment when the block
  moves, so an alignment above 16 can return a pointer away from the preserved
  contents. Alignments of 16 and below, which is what alignof() yields, are
  unaffected.