Replaced use of bool_t type with C99 bool.

Details:
- Textually replaced nearly all non-comment instances of bool_t with the
  C99 bool type. A few remaining instances, such as those in the files
  bli_herk_x_ker_var2.c, bli_trmm_xx_ker_var2.c, and
  bli_trsm_xx_ker_var2.c, were promoted to dim_t since they were being
  used not for boolean purposes but to index into an array.
- This commit constitutes the third phase of a transition toward using
  C99's bool instead of bool_t, which was raised in issue #420. The first
  phase, which cleaned up various typecasts in preparation for using
  bool as the basis for bool_t (instead of gint_t), was implemented by
  commit a69a4d7. The second phase, which redefined the bool_t typedef
  in terms of bool (from gint_t), was implemented by commit 2c554c2.

docs/BLISObjectAPI.md

@@ -53,7 +53,6 @@ The following tables list various types used throughout the BLIS object API.
 | `dim_t`           | `gint_t`                 | matrix and vector dimensions.                                        |
 | `inc_t`           | `gint_t`                 | matrix row/column strides and vector increments.                     |
 | `doff_t`          | `gint_t`                 | matrix diagonal offset: if _k_ < 0, diagonal begins at element (-_k_,0); otherwise diagonal begins at element (0,_k_). |
-| `bool_t`          | `gint_t`                 | boolean values: `TRUE` or `FALSE`.                                   |
 | `siz_t`           | `guint_t`                | a byte size or byte offset.                                          |
 
 ### Floating-point types