Change BufferIterator comparison to assert same buffer

Comparing iterators between buffers should never happen, and the
only place we did was with default constructed BufferIterator which
we replace by casting the iterator to bool.

This should improve performance on iterator heavy code.

src/buffer.hh

@@ -75,6 +75,8 @@ public:
     const char& operator[](size_t n) const noexcept;
     size_t operator- (const BufferIterator& iterator) const;
 
+    explicit operator bool() const { return static_cast<bool>(m_buffer); }
+
     BufferIterator operator+ (ByteCount size) const;
     BufferIterator operator- (ByteCount size) const;
 

src/buffer.inl.hh

@@ -104,7 +104,8 @@ inline BufferIterator::BufferIterator(const Buffer& buffer, BufferCoord coord) n
 
 inline bool BufferIterator::operator==(const BufferIterator& iterator) const noexcept
 {
-    return m_buffer == iterator.m_buffer and m_coord == iterator.m_coord;
+    kak_assert(m_buffer == iterator.m_buffer);
+    return m_coord == iterator.m_coord;
 }
 
 inline std::strong_ordering BufferIterator::operator<=>(const BufferIterator& iterator) const noexcept

src/regex.hh

@@ -39,7 +39,7 @@ struct MatchResults
     {
         SubMatch() = default;
         SubMatch(Iterator begin, Iterator end)
-            : std::pair<Iterator, Iterator>{begin, end}, matched{begin != Iterator{}}
+            : std::pair<Iterator, Iterator>{begin, end}, matched{static_cast<bool>(begin)}
         {}
 
         bool matched = false;

src/selectors.cc

@@ -483,7 +483,7 @@ select_sentence(const Context& context, const Selection& selection,
     BufferIterator first = buffer.iterator_at(selection.cursor());
     BufferIterator last;
 
-    for (++count; count > 0; --count)
+    for (int i = 0; i <= count; ++i)
     {
         if (not (flags & ObjectFlags::ToEnd) and first != buffer.begin())
         {
@@ -494,7 +494,7 @@ select_sentence(const Context& context, const Selection& selection,
                 first = prev_non_blank;
         }
 
-        if (last == BufferIterator{})
+        if (i == 0)
             last = first;
 
         if (flags & ObjectFlags::ToBegin)
@@ -552,7 +552,7 @@ select_paragraph(const Context& context, const Selection& selection,
     BufferIterator first = buffer.iterator_at(selection.cursor());
     BufferIterator last;
 
-    for (++count; count > 0; --count)
+    for (int i = 0; i <= count; ++i)
     {
         if (not (flags & ObjectFlags::ToEnd) and first.coord() > BufferCoord{0,1} and
             is_eol(*(first-1)) and first-1 != buffer.begin() and is_eol(*(first-2)))
@@ -561,7 +561,7 @@ select_paragraph(const Context& context, const Selection& selection,
                  first != buffer.begin() and (first+1) != buffer.end() and
                  is_eol(*(first-1)) and is_eol(*first))
             ++first;
-        if (last == BufferIterator{})
+        if (i == 0)
             last = first;
 
         if ((flags & ObjectFlags::ToBegin) and first != buffer.begin())