github/semantic

[#188] Fix PHP strict inequality assignment

[#186] Fix PHP string concatenation and augmented assignment

As reported in #189, the `edges` field of JSON graph output refers to
information not reflected in the rest of the output, specifically the
vertex IDs. This patch adds that information to the `ToJSON` instance
for `ControlFlowVertex`. It also includes a `toEncoding` instance for
a free speed boost.

During this patch, I realized that, because `hash` tends to return a
large number (since `Int` is 64-bit), we may run into errors when
decoding JSON. One example hash is `3500157981503982114`; passing that
to a JS engine's `Number.isSafeInteger` function returns false. The
correct thing to do here is return ids as strings, which I have done.
This is backwards-incompatible, but since this information was never
properly exposed, the impact is negligable.

We define the DiffTreeVertex protobuf message like so:

```protobuf
message DiffTreeVertex {
  int32 diff_vertex_id = 1;
  oneof diff_term {
    DeletedTerm deleted = 2;
    InsertedTerm inserted = 3;
    ReplacedTerm replaced = 4;
    MergedTerm merged = 5;
  }
}
```

This is turned into two Haskell types, a toplevel `DiffTreeVertex` type
and a `DiffTreeVertexDiffTerm` type that represents the anonymous
`oneof` type. Said types looked like so:

```haskell
data DiffTreeVertexDiffTerm
  = Deleted (Maybe DeletedTerm)
  | Inserted (Maybe InsertedTerm)
  | Replaced (Maybe ReplacedTerm)
  | Merged (Maybe MergedTerm)
  deriving stock (Eq, Ord, Show, Generic)
  deriving anyclass (Proto3.Message, Proto3.Named, NFData)
```

This is the wrong representation, as it neglects to account for the
fact that options could be added to the `diff_term` stanza. A sum type
does not provide enough constructors to handle the case of when none
of `deleted`, `inserted`, `replaced` etc. is `Just` anything. A more
correct definition follows:

```haskell
data DiffTreeVertexDiffTerm = DiffTreeVertexDiffTerm
  { deleted :: Maybe DeletedTerm
  , inserted :: Maybe InsertedTerm
  , replaced :: Maybe ReplacedTerm
  , merged :: Maybe MergedTerm
  }
```

This patch applies the above change, using `-XPatternSynonyms` to
provide backwards-compatible API shims. Though this changes JSON
output format (through the `ToJSON` instance), it should have no
bearing on backwards compatibility in protobuf objects, since there is
no way to consume diff trees as protobufs as of this writing.

Fixes #168.

This has no place here; downstream consumers can link in servant, but
we don't have any need to.

Add golden tests for `parse --proto-symbols` output.

This should help us ensure that we don't break CLI output, since our
CLI is an API.

This was made super-easy thanks to `tasty-golden`. I literally added
those two lines of code and it automatically created the reference
`.protobuf.bin` files. Slick as hell.

Quiet debug spew in specs.