mustachio: Separate out the context stack LUB type calculation

[srawlins]

I have a bigger change I've had lying around for generated renderer deduplication, and I've teased out this smaller change.

It's a no-op, and I think I've improved the documentation. I need to write docs for the tool that generates the renderers, but I hope this PR is small enough to understand, and how it is a no-op. Here are some other docs for these renderers.

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[kallentu]

This part of the codegen is kinda confusing. Why do we use the first compiler's _syntaxTree and _buildData when making the lubCompiler? Also, perhaps move this variable down there, since it's not being used for contextStacksLength anymore

All the other changes seem okay to me.

[srawlins]

First, apologies for this not being well documented, I do plan on sending out docs next.

OK, so in this section, we are looking at the "partial compilers" that have been generated for each partial file path. So if one template referred to the "_foo.html" partial with {{ >foo }} and another template also referred to that partial file with {{ >foo }}, then we have to partial compilers, and we're trying to deduplicate them. The reason they are initially separate and why we might not be able to deduplicate is that the variables in scope might be different from one calling template to another.

OK, so the variables might be different, but what is always common between these compilers (for the same partial path) is that they parse out to the same syntax tree (since its the same file on disk). And the _buildData is also just a common grab bag of data.

[kallentu]

I see! Okay, that makes a lot of sense. Yeah, some extra documentation on that would be great :)