avutil: Add functions for allocating opaque contexts for algorithms

The current API where the plain size is exposed is not of much
use - in most cases it is allocated dynamically anyway.

If allocated e.g. on the stack via an uint8_t array, there's no
guarantee that the struct's members are aligned properly (unless
the array is overallocated and the opaque pointer within it
manually aligned to some unspecified alignment).

Signed-off-by: Martin Storsjö <martin@martin.st>

doc/APIchanges

@@ -13,6 +13,10 @@ libavutil:     2011-04-18
 
 API changes, most recent first:
 
+2012-10-xx - xxxxxxx - lavu 51.43.0 - aes.h, md5.h, sha.h, tree.h
+  Add functions for allocating the opaque contexts for the algorithms,
+  deprecate the context size variables.
+
 2012-10-xx - xxxxxxx - lavf 54.18.0 - avio.h
   Add avio_closep to complement avio_close.