syntax_ext: Improve and simplify code generated by `#[global_allocator]`

syntax_ext: Improve and simplify code generated by `#[global_allocator]`

Instead of
```
mod allocator_abi { /* methods */ }
```
we now generate
```
const _: () = { /* methods */ }
```
and use `std_path` for paths referring to standard library entities.

This way we no longer need to generate `use` and `extern crate` imports, and `#[global_allocator]` starts working inside unnamed blocks.