perf: Greatly expand profiler (#38584)

Expands on #38543 (notably allows setting importance categories and
weights on tests, and a lot of internal refactoring) because I couldn't
help myself. Also allows exporting runs to json and comparing across them. See code for docs.

Release Notes:

- N/A

Cargo.lock

@@ -12169,6 +12169,9 @@ checksum = "e3148f5046208a5d56bcfc03053e3ca6334e51da8dfb19b6cdc8b306fae3283e"
 name = "perf"
 version = "0.1.0"
 dependencies = [
+ "collections",
+ "serde",
+ "serde_json",
  "workspace-hack",
 ]
 
@@ -18586,6 +18589,7 @@ dependencies = [
 name = "util_macros"
 version = "0.1.0"
 dependencies = [
+ "perf",
  "quote",
  "syn 2.0.101",
  "workspace-hack",