This method provides kj-ish interface to parse numbers.

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

one function in arena.c++ and several snprintf_result local vars
in string.c++ get optimized away because they're only used in
KJ_DASSERT() invocations.

this patch thus identifies those unused declarations with KJ_UNUSED

A compatibility layer is provided to ease migration.

Thanks to Bryan Boreham <bjboreham@gmail.com> for much help getting this started.

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

Define new KJ constants for min/max integer values and inf/nan, then remove all use of numeric_limits.

Moar type-safety, rewriting WireFormat to avoid descriptors (incomplete), decided struct pointers should point at the beginning of the struct after all.