true if L is a lexer (as specified by isLexer) and also supports the save
method as specified here; false otherwise
Specification:
The type shall support at least:
all methods and aliases specified by isLexer
L save(): returns an independent copy of the current lexer; the
copy must start at the position the original lexer 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 lexer.
A saveable lexer is a lexer enhanced with a save method analogous to the save method of ForwardRanges.