types: improve insertion and lookup performance

Currently, this uses an expensive tree lookup and requires a lock.
Switching this to a hash function instead of a comparison allows us
to make this much faster, especially for insertion.

doc/src/devdocs/types.md

@@ -236,14 +236,11 @@ MyType{Int64,2}
 
 julia> MyType{Float32, 5}
 MyType{Float32,5}
-
-julia> MyType.body.body.name.cache
-svec(MyType{Float32,5}, MyType{Int64,2}, #undef, #undef, #undef, #undef, #undef, #undef)
 ```
 
-(The cache is pre-allocated to have length 8, but only the first two entries are populated.) Consequently,
-when you instantiate a parametric type, each concrete type gets saved in a type cache.  However,
-instances containing free type variables are not cached.
+When you instantiate a parametric type, each concrete type gets saved in a type
+cache (`MyType.body.body.name.cache`). However, instances containing free type
+variables are not cached.
 
 ## Tuple types
 

src/dump.c

@@ -2924,6 +2924,9 @@ STATIC_INLINE jl_value_t *verify_type(jl_value_t *v) JL_NOTSAFEPOINT
 }
 #endif
 
+jl_datatype_t *jl_lookup_cache_type_(jl_datatype_t *type);
+void jl_cache_type_(jl_datatype_t *type);
+
 static jl_datatype_t *jl_recache_type(jl_datatype_t *dt) JL_GC_DISABLED
 {
     jl_datatype_t *t; // the type after unique'ing
@@ -2964,7 +2967,12 @@ static jl_datatype_t *jl_recache_type(jl_datatype_t *dt) JL_GC_DISABLED
         t = dt;
     }
     else {
-        t = (jl_datatype_t*)jl_cache_type_(dt);
+        t = jl_lookup_cache_type_(dt);
+        if (t == NULL) {
+            jl_cache_type_(dt);
+            t = dt;
+        }
+        assert(t->hash == dt->hash);
         assert(jl_invalid_types_equal(t, dt));
     }
     ptrhash_put(&uniquing_table, dt, t);