Reduces code size by 112 bytes on Thumb2 arch, and makes assembler faster
because comparison can be a simple equals instead of a string compare.

Not all ops have been converted, only those that were simple to convert
and reduced code size.

The chunks of memory that the parser allocates contain parse nodes and
are pointed to from many places, so these chunks cannot be relocated
by the memory manager.  This patch makes it so that when a chunk is
shrunk to fit, it is not relocated.

Make the PWM duty cycle configurable from 0.00 to 100.00 by
accepting values from 0 to 10000.
Add automatic Pin assignment when operating in PWM mode.

When scanning for devices, try reading then writing. Increase the
timeout of the transactions from 10 to 20 ms.

Properly calculate the period and the prescaler, this now allows to
set the PWM frequency down to 5Hz. Make Timer IDs go from 0 to 3.
Add the trigger definitions for the channel IRQ.

These can be used to insert arbitrary checks, polling, etc into the VM.
They are left general because the VM is a highly tuned loop and it should
be up to a given port how that port wants to modify the VM internals.

One common use would be to insert a polling check, but only done after
a certain number of opcodes were executed, so as not to slow down the VM
too much.  For example:

 #define MICROPY_VM_HOOK_COUNT (30)
 #define MICROPY_VM_HOOK_INIT static uint vm_hook_divisor = MICROPY_VM_HOOK_COUNT
 #define MICROPY_VM_HOOK_POLL if (--vm_hook_divisor == 0) { \
     vm_hook_divisor = MICROPY_VM_HOOK_COUNT;
     extern void vm_hook_function(void);
     vm_hook_function();
 }
 #define MICROPY_VM_HOOK_LOOP MICROPY_VM_HOOK_POLL
 #define MICROPY_VM_HOOK_RETURN MICROPY_VM_HOOK_POLL

- As described in the #1850.
- Add cmdline tests.

- Put the I2C bus on the corect pins
- Add the appropriate board_init to power the shield

TODO: Probably merge into vfs_fat_diskio.

If None was returned for such requests (which likely means that user simply
didn't handle them), it means successful init and default sector size of 512
bytes respectively. This makes only BP_IOCTL_SEC_COUNT a mandatory request,
and thus re-establishes parity with old interface, where only .count() is
mandatory().

This implements OO interface based on existing fsusermount code and with
minimal changes to it, to serve as a proof of concept of OO interface.

Examle of usage:

bdev = RAMFS(48)
uos.VfsFat.mkfs(bdev)
vfs = uos.VfsFat(bdev, "/ramdisk")
f = vfs.open("foo", "w")
f.write("hello!")
f.close()

We have so many configuration options, that finally having shortcuts like
this is helpful and cuts on number of ifdef's.