Various cleanups

[bjorn3]

• Be more consistent about avoiding imports of core and alloc types through mystd.
• Avoid two unnecessary allocations in handle_split_dwarf.
• Couple of other minor changes.

[bjorn3]

Just noticed that https://github.com/rust-lang/backtrace-rs/pull/666 also did most of the changes in this PR. Will rebase once that PR os merged.

[workingjubilee]

@bjorn3 rebase please?

[github-actions[bot]]

Code size changes for a hello-world Rust program linked with libstd with backtrace:

On platform ubuntu-latest:

• Original binary size: 383,480 B
• Updated binary size: 379,384 B
• Difference: -4,096 B (-1.07%)

[github-actions[bot]]

Code size changes for a hello-world Rust program linked with libstd with backtrace:

On platform ubuntu-latest:

• Original binary size: 385,800 B
• Updated binary size: 383,480 B
• Difference: -2,320 B (-0.6%)