[Feature Request] Allow memory introspection of HtmlRewriter

Allowing users to see the memory that `HtmlRewriter` used in the last rewrite cycle (`.end()` to `.end()`, basically) would be fantastic for applications who want to know more about their resource usage

I got the idea for this from [docs.rs/#930 (comment)](https://github.com/rust-lang/docs.rs/pull/930#issuecomment-668707731), having this monitoring would allow us to adjust our memory cap to whatever's realistic since we have vastly varying file sizes

Start by locating the HtmlRewriter API and its .end() implementation; the issue names no files or tests. Done means exposing memory used between one .end() call and the next so applications can inspect usage and adjust their memory cap.

Written by the indexing model from the issue text.