Inlined newSV_type(SVt_NULL) leaner than non-inlined newSV(0)

Inlined newSV_type(SVt_NULL) leaner than non-inlined newSV(0)

When a function outside of sv.c creates a SV via newSV(0):
 * There is a call to Perl_newSV
 * A SV head is uprooted and its flags set
 * A runtime check is made to effectively see if 0 > 0
 * The new SV* is returned

Replacing newSV(0) with newSV_type(SVt_NULL) should be more efficient,
because (assuming there are SV heads to uproot), the only step is:
 * A SV head is uprooted and its flags set