py: Remove mp_obj_str_builder and use vstr instead.

With this patch str/bytes construction is streamlined.  Always use a
vstr to build a str/bytes object.  If the size is known beforehand then
use vstr_init_len to allocate only required memory.  Otherwise use
vstr_init and the vstr will grow as needed.  Then use
mp_obj_new_str_from_vstr to create a str/bytes object using the vstr
memory.

Saves code ROM: 68 bytes on stmhal, 108 bytes on bare-arm, and 336 bytes
on unix x64.

py/misc.h

@@ -124,6 +124,7 @@ typedef struct _vstr_t {
 #define VSTR_FIXED(vstr, alloc) vstr_t vstr; char vstr##_buf[(alloc)]; vstr_init_fixed_buf(&vstr, (alloc), vstr##_buf);
 
 void vstr_init(vstr_t *vstr, size_t alloc);
+void vstr_init_len(vstr_t *vstr, size_t len);
 void vstr_init_fixed_buf(vstr_t *vstr, size_t alloc, char *buf);
 void vstr_clear(vstr_t *vstr);
 vstr_t *vstr_new(void);