Reduce number of allocations done by NLL
A couple of easy wins. Here are the NLL speedups that exceed 1%:
```
sentry-cli-check
avg: -3.5% min: -3.5% max: -3.5%
inflate-check
avg: -1.9% min: -1.9% max: -1.9%
inflate
avg: -1.7% min: -1.7% max: -1.7%
clap-rs-check
avg: -1.6% min: -1.6% max: -1.6%
cargo-check
avg: -1.6% min: -1.6% max: -1.6%
ripgrep-check
avg: -1.4% min: -1.4% max: -1.4%
serde-check
avg: -1.2% min: -1.2% max: -1.2%
regex-check
avg: -1.0% min: -1.0% max: -1.0%
sentry-cli
avg: -1.0% min: -1.0% max: -1.0%
```
r? @nikomatsakis