[pkg/vm] Add tool for generating DartEngine shims from entry points.

This is an initial cut of a tool for automatically creating shims for
C/C++ programs using the entry point annotations in Dart code. The tool
has two required arguments:

* the .dill file containing the kernel representation of the Dart code
* the base path of the header and implementation files to create

With a base path of 'dir/name', the header file is created as
'dir/name.h' and the implementation file is created as 'dir/name.cc'.
In addition, 'dir/name.h' is used as a basis for creating a
#ifndef/#define/#endif header guard around the header contents.


The created shims are specific to a single package, either

* a user-specified package, provided via '-p'/'--package', or
* the package of the main method

If the user does not specify a package and there is no main method
in the .dill file, the tool fails.


Each shim takes the following arguments in order when applicable:

* the isolate in which to perform the requested operation,
* the instantiated type of the generic class (for invoking, setting, or
  getting constructors, static methods, and static fields)
* the instance (for invoking, getting, or setting instance methods and
  fields)
* the type arguments (for retrieving nullable or non-nullable
  instantiated types of a generic class)
* the arguments (for invoking, setting, or getting constructors,
  methods, and fields)


The generated shims:

* cache the package library, types for non-generic classes, and
  types for generic classes instantiated with default type arguments.
  The cached persistent handles are cleared if any of the methods
  are called with a different isolate from the one used to populate
  the cache.
* automatically handle conversions between C int64_t <=> Dart int
  and C double <=> Dart double.


By default, shims are not created for allocation or initializing
uninitialized instances. To create such shims, use the '-u' command
line argument.


Currently, shims are not created for methods that take optional
or named arguments. To report an error if a shim cannot be created
for any entry points, use the '-e' command line argument.

TEST=tests/standalone/embedder_samples_test

Change-Id: Ibdf3b52d900ba98038528178485f295c5868ac9d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410500
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>