Implement array! and array_from!

[peterjoel]

Array initialiizer macros.

Usage / test cases:

• array![0,1,2,3] => [0,1,2,3]
• array![..(0..=3)] => compile error - size must be given when using spread operator
• array_from![0,1,2,3] => [0.into(), 1.into(), 2.into(), 3.into()] (and the same treatment for all examples below)
• array![1; 4] => [1, 1, 1, 1] (not [1; 4] because we don't want to require Copy)
• array![1; non_constant_var] => compile error - length must be constant
• array![0, 1; 4] => Need to decide on one of:
  • panic - not enough elements
  • compile error - not enough elements (I'm currently favouring this option)
  • [0,1,0,1] (repeat the sequence)
  • [0,1,1,1] (repeat final element)
• array![..(0..100); 4] => [0,1,2,3] (no error on too many elements when the last arg is a spread)
• array![0, 1, 2, 3, ..(0..100); 4] => panic - too many elements (the array was already "full" before the final spread)
• array![0, 1, ..(2..5), ..(0..100); 4] => panic - too many elements (the array was already "full" before the final spread)
• array![0, 1, ..(2..); 4] => [0,1,2,3]
• array![..(0..2); 4] => panic - not enough elements
• array![0, ..(1..2); 4] => panic - not enough elements

Implementation:

• This will require a little refactoring of VecInput so that its internals can be re-used.
• It should be ok for the macro to accept any expression for the length. If the expression is not constant, the generated code should produce a suitable error without extra effort.
• special cases for:
  • array![0,1,2,3] (no length specified)
  • array![1; 4] (repeat given element)
• the other variants can write to a [MaybeUninit<T>; n] from a sequence of iterators.
• If the length is larger than the number of provided elements, it often will not be possible to detect until runtime. It should:
  • panic, with a suitable error message
  • before panic, clean up the partially initialized array to avoid memory leaks.