wkb: Add missing vertex count to LineString output

The 4 byte vertex count (right after the geometry type header) was
missing from the WKB output.
wkb_test: Added vertex counts to LineString cases

The expected values for dumping linestrings to WKB were wrong; they were
missing vertex count after the header.
wkb_detail: Correct the byte composition of GCs

The description of GeometryCollections was missing 4 bytes to indicate
the number of geometries in the collection. The data was not missing
from the example WKB.
wkb: Implement __dump_geometrycollection

Some support for empty collections is still needed. Docs are also
needed.
wkb_test: Add GeometryCollectionDumpsTestCase

These tests cover 2d, 3d, and 4d test cases. Tests are still needed for
empty geometry collections, as well as strictness tests to test behavior
when geometries of different dimensions are mixed (which is not valid).