* lua: eliding repetitive NULL checks, fixes for read types, luacheck

This fixes verifier failures in some kernel versions that track whether
a possible NULL pointer has been check (and shouldn't be checked twice).

It also fixes accessing memory from maps with composite keys.

It fixes a case when temporary variable is used in condition,
and the body of the condition contains just an assignment from
immediate value to a variable that existed before the condition, e.g.:

```
local x = 1 -- Assign constant to 'x', it will be materialized here
if skb.len > 0 then -- New BB, not possible to fold as condition isn't const
    x = 2 -- Assign constant, it failed to materialize here
end -- End of BB
-- Value of x is always 1
```

The `bpf.socket()` support ljsyscall socket as first parameter, and
fixes compatibility with newer ljsyscall (type name has changed).

* reverse BPF_LD ntohl() semantics for <= 32bit loads as well

The loads using traditional instructions always do ntohl():

https://www.kernel.org/doc/Documentation/networking/filter.txt

They are however needed to support reads not using `skb`, e.g. NET_OFF

* proto: add builtins for traversing IPv6 header and extension headers

The IPv6 header is fixed size (40B), but it may contain either
extension header, or transport  protocol after it, so the caller
must check the `next_header` field before traversing to determine
which dissector to use to traverse it, e.g.:

```
if ip6.next_header == 44 then
   ip6 = ip6.ip6_opt -- Skip fragment header
end
if ip6.next_header == c.ip.proto_tcp then
   local tcp = ip6.tcp -- Finally, TCP
end
```

* reverse ntohl() for indirect BPF_LD as well

* lua: started codegen tests, direct skb support, bugfixes

This starts adding compiler correctness tests for basic expressions,
variable source tracking, and control flow.

Added:
* direct skb->data access (in addition to BPF_LDABS, BPF_LDIND)
* loads and stores from skb->data and map value backed memory
* unified variable source tracking (ptr_to_ctx, ptr_to_map[_or_null], ptr_to_skb, ptr_to_pkt, ptr_to_stack)
* BPF constants introduced between 4.10-4.15
* bpf.dump_string() to dump generated assembly (text) to string

Fixes:
* pointer nil check tracking
* dissectors for map value backed memory
* ljsyscall extensions when the version is too old
* KPRI nil variables used in conditions
* wrongly elided constant materialization on condition-less jumps
* loads/stores from stack memory using variable offset

* lua: track inverse null guards (if x ~= nil)

The verifier prohibits pointer comparisons except the first NULL check,
so both forms must be tracked, otherwise it will check for NULL twice
and verifier will reject it.

* lua: support cdata and constants larger than i32, fix shadowed variables

This adds support for numeric cdata constants (either in program, or
retrieved with GGET/UGET). Values larger than i32 are coerced into i64.

This also fixes shadowing of variables, and fixes materialization of
result of variable copy at the end of basic block.