Declaration without initialization

[sunjay]

Sometimes it's useful to declare a variable first and then initialize it later. The syntax for that would be something like what follows:

// declaration only cannot have an unspecified width
let a: [u8; 5];

a = "12345";

• [ ] appropriate static checking to ensure that variables cannot be used uninitialized. (particularly with branches and loops)
• [ ] Allocation occurs on initialization, not on declaration (so we don't allocate needlessly)
• [ ] lint for when a variable is declared but never used, initialized but never used, or updated and then never used

[sunjay]

This was mostly implemented in PR #34. The syntax is stable, though the static checking still needs work.

This may get prioritized until after MVP.