[trace] Prevent use-after-free when trace arg is a temporary.

This fixes an issue with temporary strings passed as arguments to
the trace macros. When writing a record which has a string argument,
the string's data pointer is placed in a table of trace_arg_t
values which are then passed to the function which actually does
the writing. That function will copy the contents of the string
as required. However, if the string was a temporary, it may have
already been destroyed by the time that happens.

This change first assigns the trace arguments to local variables
to ensure they remain in scope for the whole block.

TRACE_DURATION_BEGIN("foo", "name", "key", fbl::String("string"));

Tested using ASAN that the problem has been fixed.

Change-Id: I97cb382242d7736bb9f0fa6e5c9f2ae64009ba38

[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