Improve string/interpolation formatting

[Rangi42]

• The # component is renamed from "prefix" to "exact"
• The # component for type s now escapes the string characters
  • Example: "%#s" given "\"\t\\" formats as "\\"\\t\\\\"
    • This is useful for code generation: something like "MACRO foo\ndb \"%#s\", 0\nENDM" will work for any string, even containing characters like "\n" or "\0"
• The # component for type f now prints a precision suffix
  • Example: "%#.2f" given 12.34q8 formats as "12.34q8"
• The new q component specifies a precision value (valid with type f)
  • Example: "%.2q8f" given 12.34q8 formats as "12.34" even under opt Q16