Add support for unpacking values when calling variadic functions

[irh]

There's currently no way to unpack variadic arguments, which makes it impossible to forward them to another variadic function:

f = |args...|
  for i, arg in args.enumerate()
    print '{i}: {arg}'
    
g = |args...| 
  f args

f 'one', 'two', 'three'
# 0: one
# 1: two
# 2: three

g 'one', 'two', 'three'
# 0: ('one', 'two', 'three')

To solve this an unpacking operator (...) could be introduced, which would unpack values when calling a function. Variadic arguments are tuples, but ... would be more useful if it could unpack any iterable value.

f = |args...|
  for i, arg in args.enumerate()
    print '{i}: {arg}'
    
g = |args...| 
  f args... # unpack the args before calling f

g 'one', 'two', 'three'
# 0: one
# 1: two
# 2: three

x = 'abc' # Strings are iterable
g x..., 'extra' # The unpacked value doesn't have to be in last position
# 0: a
# 1: b
# 2: c
# 3: extra

A technical limitation is that currently the Call and CallInstance ops includes the argument count, but a flag could be introduced that tells the runtime to treat the arg count as a register, allowing the compiler to emit unpacking ops which increment a dynamic argument count.

See #417 for background discussion.