true if P is a parser (as specified by isLowLevelParser) and also supports the
save method as specified here; false otherwise
Specification:
The type shall support at least:
all methods and aliases specified by isLowLevelParser
P save(): returns an independent copy of the current parser; the
copy must start at the position the original parser was when this method
was called; the two copies shall be independent, in that advancing one
does not advance the other.
Checks whether its argument is a saveable parser.
A saveable parser is a parser enhanced with a save method analogous to the save method of ForwardRanges.