Adapted `trans::common::{block, fn_ctxt, scope_info}` to new naming convention.
auto merge of #7848 : michaelwoerister/rust/trans_cleanup_1, r=jdm

The following types are renamed:
```rust
block_ => Block
block => @mut Block

fn_ctx_ => FunctionContext
fn_ctx => @mut FunctionContext

scope_info => ScopeInfo
```
I also tried to convert instances of `@mut` to `&mut` or `&` but a lot of them are blocked by issue #6268, so I left it for some time later.