Add experimental `backtrace-trace-only` std feature
This experimentally allows building std with backtrace but without symbolisation. It does not affect stable and requires build-std to use. This doesn't change the backtrace crate itself so relies on the optimizer to remove the unused parts.
Example usage:
```toml
# .cargo/config.toml
[unstable]
build-std = ["core", "alloc", "panic_unwind", "std"]
build-std-features = ["backtrace", "backtrace-trace-only", "panic-unwind"]
```
```toml
# Cargo.toml
[profile.release]
opt-level = 3
lto = "thin"
codegen-units = 1
```
Ideally we should split the backtrace feature into `backtrace-trace` and `backtrace-symbolize` (with the latter dependent on the former) because Cargo features tend to work better when they're positive rather than negative. But I'm keen for this experiment not to break existing users.
cc ``@joshtriplett``
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