* Extended indexing operators
This commit adds extended indexing operators to the parser.
For instance,
```
let (.%()) p (x,y) = p.( x ).( y )
;; p.%(0,0)
```
Extended indexing operators name starts with a leading
dot "." followed by an operator symbol, except "." or "<"
and a sequence of any operators and must be closes with a
couple of enclosing parentheses ( i.e. "()", "[]", "{}" )
and then an optional assignment operator "<-":
* '.' dotsymbolchar symbolchar* '(' ')' ['<-']
* '.' dotsymbolchar symbolchar* '[' ']' ['<-']
* '.' dotsymbolchar symbolchar* '{' '}' ['<-']
Similarly, expressions of the form
* expr_1 '.' dotsymbolchar symbolchar* '(' expr_2 ')'
* expr_1 '.' dotsymbolchar symbolchar* '[' expr_2 ']'
* expr_1 '.' dotsymbolchar symbolchar* '{' expr_2 '}'
and
* expr_1 '.' dotsymbolchar symbolchar* '(' expr_2 ')' '<-' expr_3
* expr_1 '.' dotsymbolchar symbolchar* '[' expr_2 ']' '<-' expr_3
* expr_1 '.' dotsymbolchar symbolchar* '{' expr_2 '}' '<-' expr_3
are desugared to
* ('.' dotsymbolchar symbolchar* '(' ')' ) expr_1 expr_2
* ('.' dotsymbolchar symbolchar* '[' ']' ) expr_1 expr_2
* ('.' dotsymbolchar symbolchar* '{' '}' ) expr_1 expr_2
and
* ('.' dotsymbolchar symbolchar* '(' ')' '<-' ) expr_1 expr_2 expr_3
* ('.' dotsymbolchar symbolchar* '[' ']' ) expr_1 expr_2 expr_3
* ('.' dotsymbolchar symbolchar* '{' '}' '<-' ) expr_1 expr_2 expr_3