Libs: Unify concat and concat_vec

We've long had traits `StrVector` and `VectorVector` providing
`concat`/`connect` and `concat_vec`/`connect_vec` respectively. The
reason for the distinction is that coherence rules did not used to be
robust enough to allow impls on e.g. `Vec<String>` versus `Vec<&[T]>`.

This commit consolidates the traits into a single `SliceConcatExt` trait
provided by `slice` and the preldue (where it replaces `StrVector`,
which is removed.)

[breaking-change]

src/libstd/path/posix.rs

@@ -22,7 +22,7 @@ use option::Option::{None, Some};
 use kinds::Sized;
 use str::{FromStr, Str};
 use str;
-use slice::{CloneSliceExt, Splits, AsSlice, VectorVector,
+use slice::{CloneSliceExt, Split, AsSlice, SliceConcatExt,
             PartialEqSliceExt, SliceExt};
 use vec::Vec;
 
@@ -306,7 +306,7 @@ impl GenericPath for Path {
                     }
                 }
             }
-            Some(Path::new(comps.connect_vec(&SEP_BYTE)))
+            Some(Path::new(comps.as_slice().connect(&SEP_BYTE)))
         }
     }
 

src/libstd/path/windows.rs

@@ -25,8 +25,8 @@ use iter::{Iterator, IteratorExt, Map, repeat};
 use mem;
 use option::Option;
 use option::Option::{Some, None};
-use slice::SliceExt;
-use str::{SplitTerminator, FromStr, StrVector, StrExt};
+use slice::{AsSlice, SliceExt, SliceConcatExt};
+use str::{CharSplits, FromStr, Str, StrAllocating, StrPrelude};
 use string::{String, ToString};
 use unicode::char::UnicodeChar;
 use vec::Vec;