[flat.*.{syn,erasure}] Change return type of erase_if to member size_type

All other containers and container adapters use the member size_type,
and the flat maps already did so in the item specification, but not in
the synopsis.

The current wording follows the approved proposals, but this change
seems like an improvement.

Suggested by CD review feedback.

Author: tkoeppe

source/containers.tex

@@ -13244,7 +13244,8 @@
   // \ref{flat.map.erasure}, erasure for \tcode{flat_map}
   template<class Key, class T, class Compare, class KeyContainer, class MappedContainer,
            class Predicate>
-    size_t erase_if(flat_map<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred);
+    typename flat_map<Key, T, Compare, KeyContainer, MappedContainer>::size_type
+      erase_if(flat_map<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred);
 
   // \ref{flat.multimap}, class template \tcode{flat_multimap}
   template<class Key, class T, class Compare = less<Key>,
@@ -13262,8 +13263,8 @@
   // \ref{flat.multimap.erasure}, erasure for \tcode{flat_multimap}
   template<class Key, class T, class Compare, class KeyContainer, class MappedContainer,
            class Predicate>
-    size_t erase_if(flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c,
-                    Predicate pred);
+    typename flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>::size_type
+      erase_if(flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred);
 }
 \end{codeblock}
 
@@ -13286,7 +13287,8 @@
 
   // \ref{flat.set.erasure}, erasure for \tcode{flat_set}
   template<class Key, class Compare, class KeyContainer, class Predicate>
-    size_t erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred);
+    typename flat_set<Key, Compare, KeyContainer>::size_type
+      erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred);
 
   // \ref{flat.multiset}, class template \tcode{flat_multiset}
   template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
@@ -13298,9 +13300,10 @@
   template<class Key, class Compare, class KeyContainer, class Allocator>
     struct uses_allocator<flat_multiset<Key, Compare, KeyContainer>, Allocator>;
 
-  // \ref{flat.set.erasure}, erasure for \tcode{flat_multiset}
+  // \ref{flat.multiset.erasure}, erasure for \tcode{flat_multiset}
   template<class Key, class Compare, class KeyContainer, class Predicate>
-    size_t erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred);
+    typename flat_multiset<Key, Compare, KeyContainer>::size_type
+      erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred);
 }
 \end{codeblock}
 
@@ -16808,7 +16811,8 @@
 \indexlibrarymember{erase_if}{flat_set}%
 \begin{itemdecl}
 template<class Key, class Compare, class KeyContainer, class Predicate>
-  size_t erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred);
+  typename flat_set<Key, Compare, KeyContainer>::size_type
+    erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred);
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -17362,7 +17366,8 @@
 \indexlibrarymember{erase_if}{flat_multiset}%
 \begin{itemdecl}
 template<class Key, class Compare, class KeyContainer, class Predicate>
-  size_t erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred);
+  typename flat_multiset<Key, Compare, KeyContainer>::size_type
+    erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred);
 \end{itemdecl}
 
 \begin{itemdescr}