New constant caching insn: opt_getconstant_path

Previously YARV bytecode implemented constant caching by having a pair
of instructions, opt_getinlinecache and opt_setinlinecache, wrapping a
series of getconstant calls (with putobject providing supporting
arguments).

This commit replaces that pattern with a new instruction,
opt_getconstant_path, handling both getting/setting the inline cache and
fetching the constant on a cache miss.

This is implemented by storing the full constant path as a
null-terminated array of IDs inside of the IC structure. idNULL is used
to signal an absolute constant reference.

    $ ./miniruby --dump=insns -e '::Foo::Bar::Baz'
    == disasm: #<ISeq:<main>@-e:1 (1,0)-(1,13)> (catch: FALSE)
    0000 opt_getconstant_path                   <ic:0 ::Foo::Bar::Baz>      (   1)[Li]
    0002 leave

The motivation for this is that we had increasingly found the need to
disassemble the instructions between the opt_getinlinecache and
opt_setinlinecache in order to determine the constant we are fetching,
or otherwise store metadata.

This disassembly was done:
* In opt_setinlinecache, to register the IC against the constant names
  it is using for granular invalidation.
* In rb_iseq_free, to unregister the IC from the invalidation table.
* In YJIT to find the position of a opt_getinlinecache instruction to
  invalidate it when the cache is populated
* In YJIT to register the constant names being used for invalidation.

With this change we no longe need disassemly for these (in fact
rb_iseq_each is now unused), as the list of constant names being
referenced is held in the IC. This should also make it possible to make
more optimizations in the future.

This may also reduce the size of iseqs, as previously each segment
required 32 bytes (on 64-bit platforms) for each constant segment. This
implementation only stores one ID per-segment.

There should be no significant performance change between this and the
previous implementation. Previously opt_getinlinecache was a "leaf"
instruction, but it included a jump (almost always to a separate cache
line). Now opt_getconstant_path is a non-leaf (it may
raise/autoload/call const_missing) but it does not jump. These seem to
even out.

iseq.h

@@ -119,6 +119,7 @@ struct iseq_compile_data {
     int node_level;
     int isolated_depth;
     unsigned int ci_index;
+    unsigned int ic_index;
     const rb_compile_option_t *option;
     struct rb_id_table *ivar_cache_table;
     const struct rb_builtin_function *builtin_function_table;