Fix rendering problem with a superscript bigger than 10

[heeh]

The issue #38 is caused by the code at helper.rs#L126

1. superscript(n / 10).parse().unwrap_or(0) and superscript(n % 10).parse().unwrap_or(0) always fail.
2. This happens because parse function does not know the output type. It fails and we get 0 instead of ³¹ as in leetcode pick: wrong constraints #38.
3. Even if you use parse::<u32>(), parsing a superscript(e.g. ³ ) produces invalid digit error.

I believe that you want to concatenate two superscripts here.
This commit produces a valid representation.

ex) -2³¹ <= num <= 2³¹ - 1

[heeh]

This is #37.

You set up the rust toolchain correctly(https://github.com/clearloop/leetcode-cli/blob/master/rust-toolchain#L1)
However, the following workflows invoke rustc 1.54.0-nightly (35fff69d0 2021-06-06) instead of rustc 1.54.0-nightly (8cf990c9b 2021-05-15)

• https://github.com/clearloop/leetcode-cli/blob/master/.github/workflows/rust.yml#L21
• https://github.com/clearloop/leetcode-cli/blob/master/.github/workflows/clippy.yml#L21

There are two possible fixes.

1. Make workflow files explicitly use nightly-2021-05-16
2. Fix cannot find type NoneError in module std::option #37

I committed the first fix.

[clearloop]

Thanks!! @heeh! just published v0.3.7 🚀