Inconsistent escaping of directory names

[RReverser]

This is similar to #182, but I've noticed few more issues when running benchmarks. Here is a simple reproducible example:

fn bench(c: &mut Criterion) {
    c.bench_function_over_inputs("t{e.s/t", |b, i| b.iter(|| i), &[
        "a/b",
        "c{d"
    ]);
}

This creates following directory structure:

target/criterion/
├── report
├── t{e.s
│   └── t
│       └── report
└── t{e_s_t
    ├── _a_b_
    │   ├── base
    │   ├── new
    │   └── report
    └── _c{d_
        ├── base
        ├── new
        └── report

Why top-level directories for general report and parametrised reports are escaped differently? Which one is correct - is / supposed to be treated as subdirectory in benchmark names or should it be escaped? Note that it's not escaped with non-parametrised benchmark_function as well.

[RReverser]

Same for group names:

fn bench(c: &mut Criterion) {
    c.bench("g.r{ou/p", ParameterizedBenchmark::new("t{e.s/t", |b, i| b.iter(|| i), &[
        "a/b",
        "c{d"
    ]));
}

target/criterion/
├── g.r{ou
│   └── p
│       ├── report
│       └── t{e.s
│           └── t
│               └── report
├── g_r{ou_p
│   └── t{e_s_t
│       ├── _a_b_
│       │   ├── base
│       │   ├── new
│       │   └── report
│       └── _c{d_
│           ├── base
│           ├── new
│           └── report
└── report