Create a good implementation of newvec and freevec.

newvec and freevec should work properly and efficiently.

Ideally newvec (on its first call perhaps) should find
its own heap space rather than requiring the programmer
to provide one through init().

freevec must actually reclaim the chunk it is given, and
recombine it with any neighbouring free chunks so that
it is available for reallocation.

newvec should be able to find a free chunk of the appropriate
size quickly, without requiring a search of a large number
of free chunks.

Check that it works in a reasonable way, with a program
that uses memory allocations of a number of different sizes.