Add a higher level API for constructing eBPF programs

[oskardrums]

Currently ebpf_kern allows you to write eBPF programs, but it doesn't make it particularly easy or Erlang-y.
ebpf should supply some higher level API for constructing eBPF programs.

I think that the best solution would be having a fun2bpf parse-transform that takes a literal Erlang fun and outputs a sequence of bpf_instructions, but there are a few steps to cross to get there.
For starters it would be nice to have ebpf_kern functions for more common eBPF routines, like loading a value from a map to a specific register, or branching without having to count instructions for the jmp instruction.