unicorn_http: small cleanups and size reduction

unicorn_http: small cleanups and size reduction

Use Data_Make_Struct instead of Data_Wrap_Struct to avoid extra
steps/code in object allocation.  The GC will also free()
implicitly if no free callback is passed to Data_Make_Struct,
and we don't need an extra method here...

Since we're more comfortable with object management nowadays,
just make the data_get() fail with an assertion failure if it
somehow (very unlikely) ends up getting a NULL parser object.
Unicorn itself has no way of recovering other than throwing
errors to clients anyways and we have bigger problems if there's
a GC bug causing this.

Then, finally, reduce the size of our http_parser struct even
more (we'll add an int back later) since we know it's safe to
do so...