Fix various typos (#3312)

Found via `codespell -q 3 -S ./test/gtest -L "fo,pres,seh,wronly"`

test/module-test.cc

@@ -111,7 +111,7 @@ TEST(module_test, macros) {
 // but rather visibility of all client-facing overloads, reachability of
 // non-exported entities, name lookup and overload resolution within
 // template instantitions.
-// Excercise all exported entities of the API at least once.
+// Exercise all exported entities of the API at least once.
 // Instantiate as many code paths as possible.
 
 TEST(module_test, to_string) {