Do some optimisation work in GstAdapter to avoid copies in more cases.

Do some optimisation work in GstAdapter to avoid copies in more cases.

Original commit message from CVS:
* Makefile.am:
* configure.ac:
* libs/gst/base/gstadapter.c: (gst_adapter_clear),
(gst_adapter_push), (gst_adapter_peek_into), (gst_adapter_peek),
(gst_adapter_flush), (gst_adapter_take), (gst_adapter_take_buffer):
* libs/gst/base/gstadapter.h:
* tests/check/libs/adapter.c: (create_and_fill_adapter),
(GST_START_TEST), (gst_adapter_suite):
* tests/examples/Makefile.am:
Do some optimisation work in GstAdapter to avoid copies in more cases.
It could still do slightly better by merging buffers when
gst_buffer_is_span_fast is true, but is already faster.
Also, avoid traversing a single-linked list to append each incoming
buffer inside the adapter.
Add simple test app that times the adapter behaviour in different
situations, and extend the unit test to check that bytes enter and
exit the adapter in their original order.