Represent enums with regular structs; no more alignment-breaking casts.
Allow constant expressions in [Type * n].
Now actually allow using constants in those constant expressions for [T * n].
Allow expressions that are not just a single value for repeated fixed length vector syntax.
Fix pretty printer for fixed length vectors.
Add tests for const exprs in fixed vec length type and vec repeat.
Make driver::driver::parse_cfgspecs parse any variety of meta_item

All current meta items types (word, name-value, list) are now
properly parsed by rustc --cfg command line. Fixes #2399

Signed-off-by: Luca Bruno <lucab@debian.org>
auto merge of #5112 : luqmana/rust/3469, r=graydon

So this is a partial fix for #3469. Partial because it only works for simple constant expressions like `32/2` and `2+2` and not for any actual constants.

For example:
```
const FOO: uint = 2+2;
let v: [int * FOO];
```

results in:
```
error: expected constant expr for vector length: Non-constant path in constant expr
```

This seems to be because at the point of error (`typeck::astconv`) the def_map doesn't contain the constant and thus it can't lookup the actual expression (`2+2` in this case).

So, feedback on what I have so far and suggestions for how to address the constant issue?
auto merge of #5433 : lucab/rust/lucab/cfgspec, r=graydon

All current meta items types (word, name-value, list) are now
properly parsed by rustc --cfg command line. Fixes #2399
auto merge of #5356 : jld/rust/enum-less-magic, r=graydon

Fixes #1645.