unicode: String API is const byte*.

We still have that char vs byte dichotomy, but majority of string operations
now use byte.

py/lexer.c

@@ -83,8 +83,8 @@ bool str_strn_equal(const char *str, const char *strn, int len) {
 void mp_token_show(const mp_token_t *tok) {
     printf("(%d:%d) kind:%d str:%p len:%d", tok->src_line, tok->src_column, tok->kind, tok->str, tok->len);
     if (tok->str != NULL && tok->len > 0) {
-        const char *i = tok->str;
-        const char *j = i + tok->len;
+        const byte *i = (const byte *)tok->str;
+        const byte *j = (const byte *)i + tok->len;
         printf(" ");
         while (i < j) {
             unichar c = utf8_get_char(i);

py/misc.h

@@ -88,8 +88,8 @@ int m_get_peak_bytes_allocated(void);
 
 typedef int unichar; // TODO
 
-unichar utf8_get_char(const char *s);
-char *utf8_next_char(const char *s);
+unichar utf8_get_char(const byte *s);
+const byte *utf8_next_char(const byte *s);
 
 bool unichar_isspace(unichar c);
 bool unichar_isalpha(unichar c);

py/unicode.c

@@ -65,12 +65,12 @@ STATIC const uint8_t attr[] = {
     AT_LO, AT_LO, AT_LO, AT_PR, AT_PR, AT_PR, AT_PR, 0
 };
 
-unichar utf8_get_char(const char *s) {
+unichar utf8_get_char(const byte *s) {
     return *s;
 }
 
-char *utf8_next_char(const char *s) {
-    return (char*)(s + 1);
+const byte *utf8_next_char(const byte *s) {
+    return s + 1;
 }
 
 bool unichar_isspace(unichar c) {