Add classic BPF library

This library contains:
- The global constants as used by C code.
- An Insn struct that implements can generate all the BPF instructions.
- A simple BPF virtual machine implementation that can be used for
  testing programs. This has complete code-coverage and has been
  extensively fuzzed.

lib/std/x.zig

@@ -9,6 +9,7 @@ pub const os = struct {
 pub const net = struct {
     pub const ip = @import("x/net/ip.zig");
     pub const tcp = @import("x/net/tcp.zig");
+    pub const bpf = @import("x/net/bpf.zig");
 };
 
 test {