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

This reduces dependencies on stdc++. For rationale, see
  https://github.com/kamalmarhubi/capnproto/commit/62a562f6d2c7bd147958106a1cf4169533d43f83#commitcomment-14375773

Such values are not allowed by the JSON spec. We match the behavior of
JSON.stringify in Firefox and Chrome.

fixes https://github.com/sandstorm-io/capnproto/issues/261

Per spec.

This includes:
- ensuring we don't go off the end of input if it's not null-terminated
- checking for overflow and underflow
- being more careful to check that numbers match JSON lexical syntax