feat: Possible performance optimization for partial rendering

[jaredcwhite]

As reported by @luctus, rendering a partial over thousands of iterations (aka in a layout) can cause an unexpected slowdown in build time. It's possible we're not effectively caching the compiled template through Tilt prior to final render, or some other issue. FWIW, using memoization helps, aka:

<%= site.data.cached_my_partial ||= render("my_partial") %>

[jaredcwhite]

(also worth benchmarking relative to using a dedicated Component)

[jaredcwhite]

Wow, I just ran a benchmark where I loop 100,000 times and output the iteration number. Using a Ruby component + ERB template only took 1.2 seconds. Using an ERB partial took 17+ seconds! Yikes!