Change RawModule._commands to a VectorBuilder (#2839)

* Change RawModule._commands to a VectorBuilder

Use the resulting Vector to build the underlying Component's commands
and then use those instead of copying the original ArrayBuffer when
iterating on commands. Previously, the Component was using a List to
hold the commands which is particularly memory inefficient, especially
for large modules.

* Optimize Converter's handling of Seq[Command]

It previously converted the Commands to a List (which, while not
captured in the type system, they were already a List) and then used
head and tail iteration. This is less efficient with the new underlying
Vector implementation.