std: Tweak the std::env OsString/String interface

This commit tweaks the interface of the `std::env` module to make it more
ergonomic for common usage:

* `env::var` was renamed to `env::var_os`
* `env::var_string` was renamed to `env::var`
* `env::args` was renamed to `env::args_os`
* `env::args` was re-added as a panicking iterator over string values
* `env::vars` was renamed to `env::vars_os`
* `env::vars` was re-added as a panicking iterator over string values.

This should make common usage (e.g. unicode values everywhere) more ergonomic
as well as "the default". This is also a breaking change due to the differences
of what's yielded from each of these functions, but migration should be fairly
easy as the defaults operate over `String` which is a common type to use.

[breaking-change]

src/libstd/dynamic_lib.rs

@@ -101,7 +101,7 @@ impl DynamicLibrary {
     /// Returns the current search path for dynamic libraries being used by this
     /// process
     pub fn search_path() -> Vec<Path> {
-        match env::var(DynamicLibrary::envvar()) {
+        match env::var_os(DynamicLibrary::envvar()) {
             Some(var) => env::split_paths(&var).collect(),
             None => Vec::new(),
         }