Resolves an issue where lexer failed to accept CR after line continuation
character.  It also simplifies the code.

This is a temporary hack to allow othe Makefiles to build mpy-cross
recursively.

This ensures that mpy-cross is automatically built (and is up-to-date) for
ports that use frozen bytecode.  It also makes sure that .mpy files are
re-built if mpy-cross is changed.

VfsFat no longer has the listdir() method.  Rather, if listdir()
functionality is needed then one should use uos.listdir() which will call
VfsFat.ilistdir().

uos.ilistdir() is the core function, returning an iterator that yields
3-tuples.  uos.listdir() is implemented in terms of ilistdir().

Otherwise the existing FSs can interfere with the tests, and in some
cases the tests can write to the real FS on the device.

Now consistently uses the EOL processing ("\r" and "\r\n" convert to "\n")
and EOF processing (ensure "\n" before EOF) provided by next_char().

In particular the lexer can now correctly handle input that starts with CR.

Prior to this patch only 'q' and 'Q' type arrays could store big-int
values.  With this patch any big int that is stored to an array is handled
by the big-int implementation, regardless of the typecode of the array.
This allows arrays to work with all type sizes on all architectures.

This will lead to crash like:

FATAL: uncaught NLR 80a5420

On x86_32, the minimum heap size is smaller, but not 2 times, so just
use value which works for x86_64.

It's now possible to use the name "super" as a local variable.

This patch ensures that int.from_bytes only creates a big-int if necessary,
by checking the value for a small-int overflow as it's being parsed.

Don't print dupe ">>> " prompt when starting event-driven REPL.  Clear
incomplete line in transition from raw to friendly REPL.

This patch allows mounting of VFS objects right at the root directory, eg
os.mount(vfs, '/').  It still allows VFS's to be mounted at a path within
the root, eg os.mount(vfs, '/flash'), and such mount points will override
any paths within a VFS that is mounted at the root.

Memory optimizations and error handling improvements.

Versions prior to v14.0 have a bug in parsing item functions when used
within a condition: https://github.com/Microsoft/msbuild/issues/368.
Since commit [db9c2e31] this results in an error when building MicroPython
with for example VS2013.
Fix this by creating an intermediate property.

fmode.c should only be included for builds targetting 'pure' windows, i.e.
msvc or mingw builds but not when using msys or cygwin's gcc (see #2298).
Just checking if the OS is windows and UNAME doesn't have msys stil leaves
the gate open for builds with cygwin's gcc since UNAME there is e.g.
CYGWIN_NT-6.1-WOW.
Fix this by checking for 'mingw' explicitly in the compiler version; both
gcc and clang have the -dumpmachine flag so the check should be ok for
all platforms.

The with semantics of this function is close to
pkg_resources.resource_stream() function from setuptools, which
is the canonical way to access non-source files belonging to a package
(resources), regardless of what medium the package uses (e.g. individual
source files vs zip archive). In the case of MicroPython, this function
allows to access resources which are frozen into the executable, besides
accessing resources in the file system.

This is initial stage of the implementation, which actually doesn't
implement "package" part of the semantics, just accesses frozen resources
from "root", or filesystem resource - from current dir.