Change how externs are emitted for relibc

See merge request redox-os/cbindgen!4

Improve module path parsing

If a module has a path attribute use this as the module path to parse.

Pull in upstream changes

This reverts commit d78a4524.

When writing out constants the cfg attribute is not written out around
the defined constant.

Allow structs to be generated without typedefs

Allow structs, enums and unions to be declared as:

struct Name {};
typedef struct {} Name;
typedef struct Name {} Name;

Opaque enums will be declared as:
struct Name;
typedef struct Name Name;

This reverts commit 03ea684b.

Fixes #146

Fixes #141.

For projects in workspaces, the Cargo.lock file may not be in the same
folder as Cargo.toml. This patch allows the user to explicitly pass the
path to the Cargo.lock file so that this scenario can be handled.

Fixes #137.

standalone-syn 0.13.0 contains the deferred PR dtolnay/syn#363 which
allows turning off the "proc-macro" feature. This is the desired
approach to fixing the dylib dependency problem, and allows cbindgen to
once again be buildable by the release rustc compiler.

Fixes #122

Fixes #126

Fixes #119