[WIP] Build order support

[reypader]

Implemented build order to be as close as possible to what a human readable build order looks like.

[reypader]

Still marked as WIP because as of the moment, conditions are very verbose. That is, we have to specify a lot of conditions to make sure that a build order step is executed at the right time.

The current implementation in this PR, in my view is rather naive. What I'm trying to achieve is a build order that satisfies the following:

• Concise code to declare a build order
• Build order should be recoverable
  • If an attack destroys one of the required buildings in the build order, the bot should be able to rebuild it.
• Build order should be switchable
  • If the bot is doing an economic build and it scouts an aggressive build, it should be able to switch to a more aggressive build using the existing buildings