syntax_ext: format: allow multiple formats for one argument

This commit removed the restriction of only allowing one type per argument.
This is achieved by adding mappings between macro arguments and format
placeholders, then taking the mapping into consideration when emitting
the Arguments expression.

syntax_ext: format: fix implicit positional arguments

syntax_ext: format: don't panic if no args given for implicit positional args

Check the list lengths before use.
Fixes regression of `compile-fail/macro-backtrace-println.rs`.

syntax_ext: format: also map CountIsParam indices to expanded args

syntax_ext: format: fix ICE in case of malformed format args

src/test/compile-fail/ifmt-bad-arg.rs

@@ -23,9 +23,6 @@ fn main() {
     format!("{foo}", 1, foo=2);        //~ ERROR: argument never used
     format!("", foo=2);                //~ ERROR: named argument never used
 
-    format!("{0:x} {0:X}", 1);         //~ ERROR: redeclared with type `X`
-    format!("{foo:x} {foo:X}", foo=1); //~ ERROR: redeclared with type `X`
-
     format!("{foo}", foo=1, foo=2);    //~ ERROR: duplicate argument
     format!("", foo=1, 2);             //~ ERROR: positional arguments cannot follow