Pos objects are somewhat wasteful as they duplicate the origin file name and
input type for each object. on files that produce more than one Pos when parsed
this a sizeable waste of memory (one pointer per Pos). the same goes for
ptr<Pos> on 64 bit machines: parsing enough source to require 8 bytes to locate
a position would need at least 8GB of input and 64GB of expression memory. it's
not likely that we'll hit that any time soon, so we can use a uint32_t index to
locate positions instead.
- This change applies to builtins.toXML and inner workings
- Proof of concept:
```nix
let e = builtins.toXML e; in e
```
- Before:
```
$ nix-instantiate --eval poc.nix
error: infinite recursion encountered
```
- After:
```
$ nix-instantiate --eval poc.nix
error: infinite recursion encountered
at /data/github/kamadorueda/nix/poc.nix:1:9:
1| let e = builtins.toXML e; in e
|
```
because it defines _FILE_OFFSET_BITS. Without this, on
OpenSolaris the system headers define it to be 32, and then
the 32-bit stat() ends up being called with a 64-bit "struct
stat", or vice versa.
This also ensures that we get 64-bit file sizes everywhere.
* Remove the redundant call to stat() in parseExprFromFile().
The file cannot be a symlink because that's the exit condition
of the loop before.
