Enable -Wconversion: farsi.c: Solve issues.

Refactor summary:
- toF_TyB: int fn(int) --> char_u fn(int)
- put_and_redo: void fn(int) --> void fn(char_u)
    * put_curr_and_l_to_X: void fn(int) --> void fn(char_u)
        - toF_TyA: int fn(int) --> char_u fn(char_u)
    * toF_Xor_X_: int fn(int) --> char_u fn(int)
- toF_leading: int fn(int) --> char_u fn(char_u)
- toF_Rjoin: int fn(int) --> char_u fn(char_u)
- toF_ending: int fn(int) --> char_u fn(char_u)
- canF_Ljoin: bool fn(int) --> bool fn(char_u)
- canF_Rjoin: bool fn(int) --> bool fn(char_u)
- F_isterm: bool fn(int) --> bool fn(char_u)

Helped-by: oni-link <knil.ino@gmail.com>

src/nvim/CMakeLists.txt

@@ -58,7 +58,6 @@ set(CONV_SOURCES
   ex_cmds.c
   ex_docmd.c
   ex_getln.c
-  farsi.c
   fileio.c
   fold.c
   getchar.c