Struct are bundles of value types. They are essentially just shorthand
for passing around a group of individually defined values.

Struct types are declared like this:

  struct A {
    x: Smi;
    y: int32;
  }

and can be constructed explicitly like this:

  A{0, 0}

Structs can be used wherever other types are used (e.g. variables,
parameters, return values) except for parameter/return types of
builtins and runtime functions.

Struct use field access notation to set/get their values like this:

  let a: A = A{0, 0};
  let b: Smi = a.x;
  a.y = 0;

Change-Id: I9fd36a6514c37882831256a49a50809c5db75b56
Reviewed-on: https://chromium-review.googlesource.com/1122133
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54501}