Adjust the die macro to only accept ~str and to work in statement positi...

[brson]

r? @pcwalton

The error messages when failing to use ~str are bad:

<core-macros>:17:37: 17:43 error: mismatched types: expected `~str` but found `&static/str` (str storage differs: expected ~ but found &static)
<core-macros>:17                 let msgptr: &~str = &{$msg};
                                                      ^~~~~~
<core-macros>:13:4: 1:0 note: in expansion of die!
/home/brian/dev/rust/src/test/compile-fail/die-not-unique.rs:4:4: 4:17 note: expansion site
<core-macros>:17:36: 17:43 error: mismatched types: expected `&~str` but found `&&static/str` (str storage differs: expected ~ but found &static)
<core-macros>:17                 let msgptr: &~str = &{$msg};
                                                     ^~~~~~~
<core-macros>:13:4: 1:0 note: in expansion of die!
/home/brian/dev/rust/src/test/compile-fail/die-not-unique.rs:4:4: 4:17 note: expansion site
error: aborting due to 2 previous errors

[brson]

I wonder if this still works in expr position... yes, it does.

[pcwalton]

r- per IRC.