Add SvIsBOOL() macro to test for SVs being boolean-intent

These are identified as being static shared COW strings whose string
buffer points directly at PL_Yes / PL_No

sv.h

@@ -1051,6 +1051,19 @@ Remove any string offset.
 #define SvPOK_byte_pure_nogthink(sv) \
     ((SvFLAGS(sv) & (SVf_POK|SVf_UTF8|SVf_IOK|SVf_NOK|SVf_ROK|SVpgv_GP|SVf_THINKFIRST|SVs_GMG)) == SVf_POK)
 
+/*
+=for apidoc Am|BOOL|SvIsBOOL|SV* sv
+
+Returns true if the SV is one of the special boolean constants (PL_sv_yes or
+PL_sv_no), or is a regular SV whose last assignment stored a copy of one.
+
+=cut
+*/
+
+#define SvIsBOOL(sv)                            \
+    (SvIOK(sv) && SvPOK(sv) && SvIsCOW_static(sv) && \
+        (SvPVX(sv) == PL_Yes || SvPVX(sv) == PL_No))
+
 /*
 =for apidoc Am|U32|SvGAMAGIC|SV* sv