st.c: Use rb_st_* prefix instead of st_* (#2479)

The original st.c was public domain hash table implementation, but
Ruby's st.c is highly modified, and its data structure is not
compatiblie with the original one.

Therefore, when creating an extension library to wrap C code that uses
the original st.c, the symbols conflict, which leads to segfault.

This changes the prefix `st_*` of st.c functions to `rb_st_*` for
reflecting that they are specific to Ruby's, and avoid symbol conflicts.

st.c

@@ -2027,7 +2027,7 @@ st_hash_end(st_index_t h)
 
 #undef st_hash_start
 st_index_t
-st_hash_start(st_index_t h)
+rb_st_hash_start(st_index_t h)
 {
     return h;
 }