[trace] Port Fuchsia tracing system to Magenta.

The Fuchsia tracing system supports instrumentation of userspace
programs to collect trace records.

This implementation consists of...

- libtrace: a static library providing C and C++ instrumentation macros
and functions designed to be linked into client programs
- libtrace-engine: a shared library with a C ABI which manages
global tracing state in the process and provides
functions to write trace records
- libtrace-provider: a static library which implements the TraceProvider
FIDL interface and registers an instrumented
program with the trace manager
- libtrace-reader: a static library which provides a C++ API for reading
binary trace data
- unit tests
- benchmarks
- documentation

This is almost a complete rewrite of the code formerly in //apps/tracing
but it remains compatible with the trace format and preserves most of
the structure of the trace event API.

The main differences are...

- The <trace/event.h> API uses identical syntax for C and C++ with
a few extra goodies for C++ type inference.
- The trace engine is factored out as a shared library with a C ABI
instead of being written as a C++ static library, this makes it more
suitable for instrumentation of libraries and device drivers.
- The API is extensively documented.
- We actually have benchmarks to measure instrumentation overhead.

There are still a few pieces left in //apps/tracing, such as the trace
manager and command-line tools, which will be moved over later.

Change-Id: I2d2986be0e63e5aa21fe56dd03024490b0334214