Add array streaming helpers

An implementation of
#112 that
introduces a rich set of helpers for creating arrays using a
backwards-compatible approach that maintains the current semantic model
of the array/object writer being responsible for introducing plumbing.

Since the "outer" level drives the plumbing, we add writers for every
combination of value and context: `beginArray`, `beginArrayElement` and
`beginArrayMember` for creating an array in top-level, array and object
respectively and the same for other values.

In this model, `writeValue` itself performs no begin/end tracking -
instead, it is up to the entity that creates the array/object to do so
(`writeRecordValue`, `stepwiseArrayCreation` and so on).

Here's an example of writing a `writeValue` overload that writes an
array nested in an object:

```nim
proc writeValue(w: var JsonWriter, t: MyType) =
  w.beginArray()

  for i in 0 ..< t.children:
    writer.beginObjectElement()

    writer.writeMember("id", i)
    writer.writeMember("name", "item" & t.childName[i])

    writer.endObjectElement()

  writer.endArray()
```

The writing API is quite regular but requires calling special functions
depending on the context which is easy to forget - calling the wrong
variation results in invalid JSON!

The backwards-compatiblity issue can be seen in a new test case based on
a real-world example for writing byte arrays as hex strings.

Further examples are available in the documentation.

Author: arnetheduck

json_serialization/writer.nim

@@ -453,7 +453,7 @@ proc writeValue*[V: not void](w: var JsonWriter, value: V) {.raises: [IOError].}
     discard
 
   elif value is ref:
-    if value == nil:
+    if value.isNil:
       append "null"
     else:
       writeValue(w, value[])