There is no need to have helper flags in the main package. Arguably they
should live in asm if they are really needed.

Also rename Unrecognized to UnspecifiedProgram to match UnspecifiedMap.

Follow the kernel naming of map_lookup_elem more closely. Also return
an error when a key is not found, instead of the awkward boolean.

Use the IsNotExist infrastructure for Delete calls and remove DeleteStrict.

Let the user specify flags instead of adding a new function for every
flag of map_update_elem. This reduces the API surface, and makes the
library more versatile. It also provides a clear way to support
BPF_F_LOCK which wasn't possible previously.

This gives us flexibility to add more detailed errors later.

Clarify the use case of MapABI and ProgramABI, and document the behaviour
of Check on the function instead of on the struct.

Marshaler isn't really part of the API, it's just there for
documentation purposes. Let's improve the documentation instead of
cluttering the API.

The current receive is called bpf, which doesn't make much sense in
the context of the package.

Lookup, Update, Delete and NextKey are probably the most performance
critical functions. Add a small benchmark for them. On my machine
I get the following:

    name           time/op
    Map/Lookup-8   622ns ± 1%
    Map/Update-8   650ns ± 1%
    Map/NextKey-8  540ns ± 0%
    Map/Delete-8   640ns ± 3%

    name           alloc/op
    Map/Lookup-8   0.00B
    Map/Update-8   0.00B
    Map/NextKey-8  0.00B
    Map/Delete-8   40.0B ± 0%

    name           allocs/op
    Map/Lookup-8    0.00
    Map/Update-8    0.00
    Map/NextKey-8   0.00
    Map/Delete-8    2.00 ± 0%

Delete allocates because it returns a not found error when deleting
a non-existing key.

eBPF helpers are added frequently, so we should make updating the list
simple. Remove the (duplicate, probably incorrect) documentation and
re-generate the new list of functions.

XAdd is just another mode of the Store class, so keep the
naming consistent.

The test was used to show the difference between old and new
syntax. Remove it since it's not needed anymore.