core: string: add UTF-8 and null-char support; and some new string functions

b_string now uses UTF-8 internally, and can correctly manipulate strings
that contain non-ASCII and multi-byte codepoints.

b_string now tracks the length of a string in both bytes and unicode codepoints.

string insertion functions have been updated to correctly handle strings with
multi-byte codepoints, so the index parameter of each function now refers to codepoints
rather than bytes. inserting single-byte chars into a string with no multi-byte codepoints
is still optimised to used array indexing and memmove.

a b_string_iterator has been added to simplify iterating through a UTF-8 string, without
having to use a charAt()-style interface that would incur performance penalties.

strings can now also contain null bytes.

new functions include:
  - b_string_tokenise: a b_iterator interface for iterating through tokens
    in a string. similar to strtok except that:
    * it is re-entrant, and uses no global state.
    * it supports delimiters that are longer than one character and/or contain
      multi-byte UTF-8 codepoints.
    * it doesn't modify the string that is being iterated over.
    * it correctly handles strings with multi-byte UTF-8 codepoints and null chars.
  - b_string_compare: for comparing strings. necessary to use this rather than strcpy
    as b_strings can now contain null chars.

object/string.h

@@ -8,9 +8,14 @@
 
 struct b_string {
 	struct b_object s_base;
-	/* length of string, not including null-terminator */
+	/* length of string in bytes, not including null-terminator.
+	 * a multi-byte utf-8 codepoint will be counted as multiple bytes here */
 	unsigned int s_len;
-	/* maximum length of string storable in the currently-allocated buffer, not including null terminator */
+	/* length of string in codepoints, not including null-terminator.
+	 * a multi-byte utf-8 codepoint will be counted as one codepoint here */
+	unsigned int s_codepoints;
+	/* maximum length of string storable in the currently-allocated buffer
+	 * in bytes, not including null terminator */
 	unsigned int s_max;
 	union {
 		char d_inline[STRING_INLINE_CAPACITY + 1];