fix: qualify std::exp after including <cmath>

As noted in #462, including <cmath> only guarantees that std::exp is
declared, so calling exp(double) unqualified is not portable.

Squashed commit with a style change to the original PR:

commit 8b9d48c0340f3fb82de59a6f7ec71ddfd86a602f
Author: Chen Gong <chen.sst@gmail.com>
Date:   Sun Jan 16 00:49:47 2022 +0800

commit df4fe3e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Aug 10 10:18:38 2021 +0100

Closes #476

src/rime/gear/script_translator.cc

@@ -488,9 +488,9 @@ void ScriptTranslation::PrepareCandidate() {
                        start_,
                        start_ + user_phrase_code_length,
                        entry);
-    cand->set_quality(exp(entry->weight) +
-                      translator_->initial_quality() +
-                      (IsNormalSpelling() ? 0.5 : -0.5));
+    cand->set_quality(std::exp(entry->weight) +
+                     translator_->initial_quality() +
+                     (IsNormalSpelling() ? 0.5 : -0.5));
   }
   else if (phrase_code_length > 0) {
     DictEntryIterator& iter = phrase_iter_->second;
@@ -502,9 +502,9 @@ void ScriptTranslation::PrepareCandidate() {
                        start_,
                        start_ + phrase_code_length,
                        entry);
-    cand->set_quality(exp(entry->weight) +
-                      translator_->initial_quality() +
-                      (IsNormalSpelling() ? 0 : -1));
+    cand->set_quality(std::exp(entry->weight) +
+                     translator_->initial_quality() +
+                     (IsNormalSpelling() ? 0 : -1));
   }
   candidate_ = cand;
 }

src/rime/gear/table_translator.cc

@@ -81,10 +81,10 @@ an<Candidate> TableTranslation::Peek() {
   if (phrase) {
     phrase->set_comment(comment);
     phrase->set_preedit(preedit_);
-    phrase->set_quality(exp(e->weight) +
-                        options_->initial_quality() +
-                        (incomplete ? -1 : 0) +
-                        (is_user_phrase ? 0.5 : 0));
+    phrase->set_quality(std::exp(e->weight) +
+                       options_->initial_quality() +
+                       (incomplete ? -1 : 0) +
+                       (is_user_phrase ? 0.5 : 0));
   }
   return phrase;
 }