Forward slashes don't need to be escaped in JSON.

In particular, it should not be necessary to specify a void union member's value explicitly. The discriminator should be sufficient. It should also be permitted to omit default values as long as the descriminator specifies the variant.

(Also make sure that external struct-unions are tested.)

This is particularly needed since JsonValueHandler is re-registered every time a JsonValue field is seen.

The code was init()ing the union field when the discriminant was seen, but this only works if the field type is a struct or group. Instead we store the field schema and initialize it when we see the value later.

This avoids the need for a weird struct-as-namespace pattern inside json.capnp.

Features:
- Rename any field or enum value for JSON purposes.
- Flatten structs/groups into their parent object, possibly with a prefix.
- Assign a special discriminant field for unions, so that union members can be flattened without ambiguity.

Simplifiy implementation and, as a bonus, allow overflowing floating numbers to convert to +/-Infinity like they do in JSON.parse.

Also update outdated comment about NaN and Infinity being encoded as `null`.

 - Add support for decoding with Orphanage.
 - Allow decoding of non-struct root JSON objects.
 - Add support for custom decoding type handlers.
 - Add support for custom decoding field handlers.

This reverts commit f836a5fc.

This method provides kj-ish interface to parse numbers.

Moves code from capnp json decoder to kj::StringPtr.

Fix JSON handler for DynamicStruct. The handler needs to be told the struct's schema to construct an orphan.

Also improve error message when addTypeHandler() is given a handler for a dynamic type but no specific schema is specified. We could support this eventually but we don't presently.

This allows to preserve these values in capnp -> json -> capnp conversion.

This encapsulates the input array, hiding it from the parser's methods.
Access is forced to go through `advance` and `consume*` methods on the
new `Input` object. The aim is to make it harder to have errors like
https://github.com/sandstorm-io/capnproto/pull/270