Rescue master branch PRs: Template enhancements

[GuySartorelli]

Requires
#10507

Rescues

• API: Preserve variable types in template lookup args (closes #4822) #8152
• Add support for null values in template lookups #8456
• API: <% loop %> and <% with %> only ever create one new scope level (fixes #4015) #8148

I have opted to keep these in one PR because two of them touch the template parser which needs to be recompiled whenever the .peg file is changed - it ends up being a lot easier to just do these all in one go.

Parent issue

• Rescue master branch commits #10350