Introduce til::rle - a run length encoded vector

## Summary of the Pull Request

Introduces `til::rle`, a vector-like container which stores elements of
type T in a run length encoded format. This allows efficient compaction
of repeated elements within the vector.

## References

* #8000 - Supports buffer rewrite work. A re-use of `til::rle` will be
  useful as a column counter as we pursue NxM storage and presentation.
* #3075 - The new iterators allow skipping forward by multiple units,
  which wasn't possible under `TextBuffer-/OutputCellIterator`.
  Additionally it also allows a bulk insertions.
* #8787 and #410 - High probability this should be `pmr`-ified
  like `bitmap` for things like `chafa` and `cacafire`
  which are changing the run length frequently.

## PR Checklist

* [x] Closes #8741
* [x] I work here.
* [x] Tests added.
* [x] Tests passed.

## Validation Steps Performed

* [x] Ran `cacafire` in `OpenConsole.exe` and it looked beautiful
* [x] Ran new suite of `RunLengthEncodingTests.cpp`

Co-authored-by: Michael Niksa <miniksa@microsoft.com>

.github/actions/spelling/allow/allow.txt

@@ -1,6 +1,7 @@
 Apc
 apc
 clickable
+copyable
 dalet
 Dcs
 dcs

src/buffer/out/AttrRow.hpp

@@ -20,16 +20,17 @@ Revision History:
 
 #pragma once
 
-#include "TextAttributeRun.hpp"
-#include "AttrRowIterator.hpp"
+#include "til/rle.h"
+#include "TextAttribute.hpp"
 
 class ATTR_ROW final
 {
+    using rle_vector = til::small_rle<TextAttribute, uint16_t, 1>;
+
 public:
-    using const_iterator = typename AttrRowIterator;
+    using const_iterator = rle_vector::const_iterator;
 
-    ATTR_ROW(const UINT cchRowWidth, const TextAttribute attr)
-    noexcept;
+    ATTR_ROW(uint16_t width, TextAttribute attr);
 
     ~ATTR_ROW() = default;
 
@@ -39,28 +40,13 @@ class ATTR_ROW final
     noexcept = default;
     ATTR_ROW& operator=(ATTR_ROW&&) noexcept = default;
 
-    TextAttribute GetAttrByColumn(const size_t column) const;
-    TextAttribute GetAttrByColumn(const size_t column,
-                                  size_t* const pApplies) const;
-
-    size_t GetNumberOfRuns() const noexcept;
-
-    size_t FindAttrIndex(const size_t index,
-                         size_t* const pApplies) const;
-
-    std::vector<uint16_t> GetHyperlinks();
-
-    bool SetAttrToEnd(const UINT iStart, const TextAttribute attr);
-    void ReplaceAttrs(const TextAttribute& toBeReplacedAttr, const TextAttribute& replaceWith) noexcept;
-
-    void Resize(const size_t newWidth);
-
-    [[nodiscard]] HRESULT InsertAttrRuns(const gsl::span<const TextAttributeRun> newAttrs,
-                                         const size_t iStart,
-                                         const size_t iEnd,
-                                         const size_t cBufferWidth);
+    TextAttribute GetAttrByColumn(uint16_t column) const;
+    std::vector<uint16_t> GetHyperlinks() const;
 
-    static std::vector<TextAttributeRun> PackAttrs(const std::vector<TextAttribute>& attrs);
+    bool SetAttrToEnd(uint16_t beginIndex, TextAttribute attr);
+    void ReplaceAttrs(const TextAttribute& toBeReplacedAttr, const TextAttribute& replaceWith);
+    void Resize(uint16_t newWidth);
+    void Replace(uint16_t beginIndex, uint16_t endIndex, const TextAttribute& newAttr);
 
     const_iterator begin() const noexcept;
     const_iterator end() const noexcept;
@@ -69,17 +55,14 @@ class ATTR_ROW final
     const_iterator cend() const noexcept;
 
     friend bool operator==(const ATTR_ROW& a, const ATTR_ROW& b) noexcept;
-    friend class AttrRowIterator;
     friend class ROW;
 
 private:
     void Reset(const TextAttribute attr);
 
-    boost::container::small_vector<TextAttributeRun, 1> _list;
-    size_t _cchRowWidth;
+    rle_vector _data;
 
 #ifdef UNIT_TESTING
-    friend class AttrRowTests;
     friend class CommonState;
 #endif
 };