Log Message:
libixion, py-libixion: Update to 0.18.1

Changelog:
0.81.1
* fixed a 32-bit Linux build issue as discovered on Debian, due to
  a clash on two 32-bit unsigned integer types being used with std::variant.

0.18.0
C++ API
* removed the formula_model_access interface from model_context, and
  switched to using model_context directly everywhere.
* revised formula_tokens_t type to remove use of std::unique_ptr for each
  formula_token instance. This should improve memory locality when iterating
  through an array of formula token values. A similar change has also been
  made to lexer_tokens_t and lexer_token types.

formula interpreter
* added the following 41 built-in functions:
    ABS
    AND
    COLUMN
    COLUMNS
    COUNT
    COUNTBLANK
    EXACT
    FALSE
    FIND
    ISBLANK
    ISERROR
    ISEVEN
    ISFORMULA
    ISLOGICAL
    ISNA
    ISNONTEXT
    ISNUMBER
    ISODD
    ISREF
    ISTEXT
    LEFT
    MEDIAN
    MID
    MODE
    N
    NA
    NOT
    OR
    REPLACE
    REPT
    RIGHT
    ROW
    ROWS
    SHEET
    SHEETS
    SUBSTITUTE
    T
    TEXTJOIN
    TRIM
    TRUE
    TYPE

* added support for multi-sheet references in Excel A1 and Excel R1C1
  grammers.