Move the formatter functions into separate files so that display.c is less of a monolith; and replace the ctype.h macros with internal versions.

src/include/pv.h

@@ -54,6 +54,11 @@ typedef enum {
  * Simple string functions for processing numbers.
  */
 
+/*
+ * Return true if the character is an ASCII digit.
+ */
+extern bool pv_isdigit(char);
+
 /*
  * Return the given string converted to a double, for use as a time
  * interval.
@@ -120,6 +125,11 @@ extern size_t pv_strlcat(char *, const char *, size_t);
  */
 extern size_t pv_strwidth(const char *, size_t);
 
+/*
+ * Return true if the character is printable.
+ */
+extern bool pv_isprint(char);
+
 /*
  * Functions relating to elapsed time.
  */