Add "default value" statements

[scizzorz]

The implementation was a little trickier than I intended because the behavior was a little trickier than I expected. The current details for handling variable assignments are roughly as follows:

• A Mold is created as the main execution context with a variable map containing a few special Mold variables like MOLD_ROOT and MOLD_DIR
• A Moldfile is created from a source file and collects all of the top-level variable assignments into its own variable map
• Each Recipe in a Moldfile collects all of its internal variable assignments into its own variable map
• When the Mold opens a Moldfile, such as the initial file or during an import, all of the Moldfile's contents (recipes, variables, etc) are merged into the Mold, overriding any existing values.
• When a Recipe is executed, the recipe's contents (working dir, variables, etc) are merged into a copy of the the Mold's contents, overriding any existing values.
• When variable expansion takes place, variables are looked up in the Recipe's execution variable map and fall back to the OS environment.
• Variable assignments unconditionally override previous assignments, meaning the latest assignment takes priority.

So, in short, the variable priority is roughly: Recipe variables > Moldfile variables > environment variables. Because of this ordering, it's not possible for Mold to provide a default value for a variable that can be overridden by the environment; any default values will have to be handled in the script rather than in Mold.

The open issue #128 is intended to address this, and I was expecting that I could simply parse the default assignment and apply it if the variable map and environment turned up empty. Because of the multi-layered variable system (Mold, Moldfile, and Recipe), my initial assumptions don't work out well without breaking some boundaries: a Moldfile would need to inspect the variable map of the Mold that's opening it, and a Recipe would need to inspect the variable map of the Moldfile that defines it, etc. Unfortunately, I don't think that's feasible and would significantly impact the interfaces between those concepts.

The approach I've taken here is to add a secondary variable map, defaults, for all three structures. This variable map takes the opposite prioritization approach: the first definition takes priority, rather than the last. That is:

var foo = "one"
var foo = "two"

$foo would evaluate to "two" here (last assignment takes priority), while:

default foo = "one"
default foo = "two"

$foo would evaluate to "one" here (first assignment takes priority).

Following the way that Recipes are treated as happening "after" the Moldfile, allowing their variables override the file's, but respecting the inversion of priority for default variables, the file's defaults override the recipe's defaults. Default values are appended to the priority list after environment variables, making the final priority list look something like this: Recipe variables > Moldfile variables > environment variables > Moldfile defaults > Recipe defaults.

[scizzorz]

The PR is updated with some more behavioral changes. Notably:

• Recipes can no longer have variables or defaults
  • I wasn't using them anywhere in ~600 lines of moldfiles and they added a good amount of complexity
• Variable expansion is delayed until just before use
  • Variable definition order is retained, however
  • This way a working directory can include a variable that may be defined in another location
• When processing a Moldfile, it now has a reference to the owning Mold and can check for a variable's existence. This breaks the interface boundary a bit, but it avoids weird ordering issues that showed up during the initial approach. I think it's the most effective way to handle this.