Two fixes for build failure on Android (resubmit)

Have not found a universal way to detect C11 and after some deliberation
__BIONIC__ seems to be a better choice for these cases.

Fix build error because DTTOIF function is not available.

Reorder ifdef checks in BTreeImpl::growTree to make posix_memalign the
default case and hide aligned_alloc behind feature test macro. This
covers more systems and actually fixes the compatibility with Android.

Make Canceler::isEmpty() const

Fixups for 0.7

capnp-js-base has moved

Fix linking during lite builds

More JSON annotation fixes/improvements

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.

Implement annotations to control JSON parsing.

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.

Disallow copy construction/assignment of (Atomic)Refcounted classes