Buffer: Remove m_line_count field from BufferIterator

It seems unlikely this would give performance gain, as buffer
lines are always accessed when we read that field, leading to
all the necessary data already being in memory. Removing it
reduces the size of a BufferIterator, which are already pretty
hefty objects.

src/buffer.hh

@@ -92,7 +92,6 @@ public:
 private:
     SafePtr<const Buffer> m_buffer;
     BufferCoord m_coord;
-    LineCount m_line_count;
     StringView m_line;
 };
 

src/buffer.inl.hh

@@ -100,7 +100,6 @@ inline BufferCoord Buffer::end_coord() const
 
 inline BufferIterator::BufferIterator(const Buffer& buffer, BufferCoord coord) noexcept
     : m_buffer{&buffer}, m_coord{coord},
-      m_line_count{buffer.line_count()},
       m_line{coord.line < buffer.line_count() ? (*m_buffer)[coord.line] : StringView{}} {}
 
 inline bool BufferIterator::operator==(const BufferIterator& iterator) const noexcept
@@ -183,7 +182,7 @@ inline BufferIterator& BufferIterator::operator++()
 {
     if (++m_coord.column == m_line.length())
     {
-        m_line = (++m_coord.line < m_line_count) ?
+        m_line = (++m_coord.line < m_buffer->line_count()) ?
             (*m_buffer)[m_coord.line] : StringView{};
         m_coord.column = 0;
     }