SvPV_shrink_to_cur: don't be unrealistic, do include space for COW

The `SvPV_shrink_to_cur` macro shrinks an allocation to `SvCUR(sv) + 1`,
which does not include an additional byte for Copy-On-Write (COW).

GH#22116 - a902d92 - short-circuited
constant folding on CONST OPs, as this should be unnecessary. However,
Dave Mitchell noticed that it had the inadvertent effect of disabling
COW on SVs holding UTF8 string literals (e.g. `"\x{100}abcd"`).

When a CONST OP is created, `Perl_ck_svconst` should mark its SV as
being COW-able. But SVs built via `S_scan_const`, when that has
called `SvPV_shrink_to_cur`, have resulting `SvLEN(sv)` values that
fail the `SvCANCOW(sv)` test. Previously, constant folding had the
effect of copying the literal into a buffer large enough for COW.

This commit modifies `SvPV_shrink_to_cur` so that it adds an
additional byte to allow for subsequent direct COWing.

The macro has also been modified to use the new `expected_size` macro,
so that reallocation will not be attempted if the resulting buffer
size is not going to be any smaller.

The intended saving is compared with SvLEN(sv), rather than SvCUR(sv),
as this also helps in cases such as SvCUR(sv) having previously been
set to zero and a size smaller than PERL_STRLEN_NEW_MIN is requested.
This will also enable some size checks at call sites to be simplified.

Author: richardleach

sv.h

@@ -1590,9 +1590,22 @@ L</C<SV_CHECK_THINKFIRST_COW_DROP>> before calling this.
 =cut
 */
 
-#define SvPV_shrink_to_cur(sv) STMT_START { \
-                   const STRLEN _lEnGtH = SvCUR(sv) + 1; \
-                   SvPV_renew(sv, _lEnGtH); \
+/* Notes: Ensure the buffer is big enough to be COWed in the future, so
+          + 1 for the trailing null byte + 1 for the COW count.
+ * The `expected_size` call will, at worst, ensure that the buffer size
+ * is no smaller than the expected minimim allocation and that the given
+ * size is rounded up to the closest PTRSIZE boundary. Depending on
+ * per-malloc implementation, it might return the exact size that would
+ * be allocated for the specified _lEnGtH. If the return value from
+ * `expected_size` is not smaller than the current buffer allocation,
+ * there is no point in calling SvPV_renew.
+*/
+
+#define SvPV_shrink_to_cur(sv) STMT_START {                       \
+                   const STRLEN _lEnGtH = SvCUR(sv) + 2;          \
+                   const STRLEN _eXpEcT = expected_size(_lEnGtH); \
+                   if (SvLEN(sv) > _eXpEcT)                       \
+                       SvPV_renew(sv, _eXpEcT);                   \
                  } STMT_END
 
 /*