165: Use local_inner_macros to resolve all helper macros within $crate r=Dylan-DPC a=dtolnay

This fixes the following error when using Rust 2018 style macro imports.

```rust
use bitflags::bitflags;
```

```console
error: cannot find macro `__bitflags!` in this scope
  --> src/main.rs:5:1
   |
5  | / bitflags! {
6  | |     struct Flags: u32 {
7  | |         const A = 0b00000001;
8  | |         const B = 0b00000010;
...  |
11 | |     }
12 | | }
   | |_^ help: you could try the macro: `bitflags`
   |
```

The `local_inner_macros` modifier resolves all macro invocations made from within that macro as helpers in the same crate. So if `bitflags!` expands to an invocation of `__bitflags!` then this would be resolved as `$crate::__bitflags!` rather than requiring the caller to have `__bitflags` in scope.

The attribute is ignored by pre-2018 compilers so bitflags will continue to work as normal with #[macro_use].

In the future when dropping compatibility with pre-2018 compilers we can remove the `local_inner_macros` modifier and use our own explicit `$crate::` prefixes on invocations of helper macros.

Co-authored-by: David Tolnay <dtolnay@gmail.com>

I thought I needed it for Commit::parent_count(); turns out that I
didn't, but since I've tested the update anyway I thought I'd include it.

I had to delete Cargo.lock and let cargo regenerate it, since otherwise
I got an error that I think was fixed in a newer bitflags version:
bitflags/bitflags#165