Implement multiple comma-separated variable declarations in 'my' statement

[masak]

Like this:

my temperature = 37,
   pressure = 19,
   balloonCount = 65;

Note that this is incompatible with both Perl 5/6 and Python 2/3. Both of these families of languages prefer what we might call "parallel assignment" when declaring:

my ($temperature, $pressure, $balloonCount) = 37, 19, 65;    # Perl
temperature, pressure, balloonCount = 37, 19, 65      # Python

We find a friend in JavaScript, though, and model this feature on it:

var temperature = 37,
    pressure = 19,
    balloonCount = 65;

The biggest difference in practice is that in serial assignment, earlier variables in the same declaration can already be used when assigning to later ones:

my temperature = 37,
   pressure = 19,
   ratio = temperature / pressure;    # didn't even try to get the units right

Oh, and it's also fine to omit the assignment, as usual, and get None.

We're allowed — encouraged, even — to create any kind of API for declaring variables to make this work. It's considered a part of the challenge that we use the my keyword even for this new serial-assignment declaration. Of course declaring only one variable should still work rather than, say, cause a parsing rule conflict.

[masak]

The biggest difference in practice is that in serial assignment, earlier variables in the same declaration can already be used when assigning to later ones

Cf parameter defaults in #112.

[masak]

#279 (correctly) states that if we went ahead and made my into a term, then this issue would become kinda moot. More exactly, this is now definitely a compile-time error:

my a = 1, b = 2;

(That b there is undeclared.)

Closing.