Speed up ParseSymbol by avoiding allocations

[varungandhi-src]

TODO cleanups - remove stale benchmarks/update them

Test plan

TODO

[varungandhi-src]

Should take UTF8String as input

[kritzcreek]

I tried running the benchmarks against chromium-1.scip (to compare parsing the same symbols), but it looks like the parser is failing to parse the symbols:

@@ -175,7 +175,11 @@ func TestUtf8Validation(t *testing.T) {
 		var sym Symbol
 		for i := 0; i < b.N; i++ {
 			occ := allOccurrences[i]
-			_ = parsePartialSymbolV2(occ.Symbol, true, &sym)
+			err = parsePartialSymbolV2(occ.Symbol, true, &sym)
+			if err != nil {
+				panic(fmt.Sprintf("Failed to parse '%s' with %s", occ.Symbol, err))
+				// fmt.Printf("Failed to parse '%s' with %s", occ.Symbol, err)
+			}
 		}
 	}
 	stdUtf8ValidationOnly := func(b *simpleBenchmark) {

=== RUN   TestUtf8Validation
--- FAIL: TestUtf8Validation (17.96s)
panic: Failed to parse 'cxx . todo-pkg todo-version `apps/switches.h:6:9`!' with unrecognized descriptor "do-pkg " [recovered]
	panic: Failed to parse 'cxx . todo-pkg todo-version `apps/switches.h:6:9`!' with unrecognized descriptor "do-pkg "