Make gv_stashpvn() use PL_stashcache

perl has a stash name lookup cache, which is currently just used for
looking up up class names in Some::Class->method calls. This means
that a lookup of the class name 'A::B::C' gets reduced from doing
several stash lookups (e.g. $::{'A::'}{'B::'}{'C::'}) to a single
cache lookup (e.g. $PL_stashcache{'A::B::C::'}, so to speak).

Make gv_stashpvn() use this cache too, which means that all package
name lookups benefit, not just class method calls. Among other things,
this also indirectly affects bless operations both from Perl (OP_BLESS)
and XS.

gv.c

@@ -1313,8 +1313,8 @@ The most important of which are probably GV_ADD and SVf_UTF8.
 =cut
 */
 
-HV*
-Perl_gv_stashpvn(pTHX_ const char *name, U32 namelen, I32 flags)
+PERL_STATIC_INLINE HV*
+S_stashpvn(pTHX_ const char *name, U32 namelen, I32 flags)
 {
     char smallbuf[128];
     char *tmpbuf;
@@ -1351,6 +1351,25 @@ Perl_gv_stashpvn(pTHX_ const char *name, U32 namelen, I32 flags)
     return stash;
 }
 
+HV*
+Perl_gv_stashpvn(pTHX_ const char *name, U32 namelen, I32 flags)
+{
+    HV* stash;
+    const HE* const he = (const HE *)hv_common(
+        PL_stashcache, NULL, name, namelen,
+        (flags & SVf_UTF8) ? HVhek_UTF8 : 0, 0, NULL, 0
+    );
+    if (he) return INT2PTR(HV*,SvIVX(HeVAL(he)));
+
+    stash = S_stashpvn(aTHX_ name, namelen, flags);
+    if (stash && namelen) {
+        SV* const ref = newSViv(PTR2IV(stash));
+        hv_store(PL_stashcache, name,
+            (flags & SVf_UTF8) ? -(I32)namelen : (I32)namelen, ref, 0);
+    }
+    return stash;
+}
+
 /*
 =for apidoc gv_stashsv