[uniset] Crash when testing for non-first range

[zbraniecki]

use icu_unicodeset::UnicodeSet;
use icu_unicodeset::UnicodeSetBuilder;
use std::char;

fn main() {
    let ranges = vec!['A'..='Z', 'a'..='z'];
    let mut builder = UnicodeSetBuilder::new();

    for range in &ranges {
        builder.add_range(range);
    }

    let set = builder.build();

    assert_eq!(set.contains_range(&('c'..='e')), true);
}

crashes with:

thread 'main' panicked at 'index out of bounds: the len is 4 but the index is 4', /Users/zbraniecki/projects/icu4x/components/uniset/src/uniset.rs:189:36

full stack:

thread 'main' panicked at 'index out of bounds: the len is 4 but the index is 4', /Users/zbraniecki/projects/icu4x/components/uniset/src/uniset.rs:189:36
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: std::panicking::rust_panic_with_hook
   6: rust_begin_unwind
   7: core::panicking::panic_fmt
   8: core::panicking::panic_bounds_check
   9: <usize as core::slice::SliceIndex<[T]>>::index
  10: core::slice::<impl core::ops::index::Index<I> for [T]>::index
  11: <alloc::vec::Vec<T> as core::ops::index::Index<I>>::index
  12: icu_unicodeset::uniset::UnicodeSet::contains_range
  13: uniset_test::main
  14: std::rt::lang_start::{{closure}}
  15: std::rt::lang_start_internal
  16: std::rt::lang_start
  17: main

[EvanJP]

I apologize for the bug:

Further outlined in fix #269 , there was an off by one error that I failed to account for in my testing. Thanks for pointing it out! Should be merged tomorrow.