Extend totalSizeInWords() to also return a count of capabilities, which helps when a separate capability table needs to be allocated as well.  Use this in the RPC system.

Get rid of 'Advanced' server methods, just have one method that takes a CallContext.  A typedef reduces boilerplate.

Revamp concurrency model, part 1:  EventLoop no longer allows cross-thread event queuing, simplifying many things.  Capability clients are no longer thread-safe, so they don't have to be so const.  In the future, explicit ways to communicate between threads will be re-added, but threads will be treated more like separate vats that just happen to have a particularly fat pipe.  Upcoming:  Remove mutexes.

Don't rely on default assignment operators for cap clients since they break with double virtual inheritance.

Remove 'has' for primitive fields.  Dynamic version always returns true.  This also means that primitives are always included in text format, even if they are default-valued.

Fix bug handling absolute source paths with no --src-prefix -- just output to the same directory in this case.

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

Interface code generation.  Also update method grammar to allow multiple returns, and allow specifying request/response structs instead of parameter lists.

Extend interface grammar to support inheritance, and update compiler to properly compile interfaces.

Revamp generated interface to Object fields.  Now there is an ObjectPointer::{Reader,Builder}.  This will simplify the dynamic API (next change) and also makes it easier to delegate decisions about the object type to a function that doesn't know about the context where the object lives (i.e. by passing an ObjectPointer::Builder).

Factor Pointer{Reader,Builder} out from {Struct,List}{Reader,Builder}.  Will make it easier to deal with Object fields.