small fix to case of variables (Rust coding convention)

ref. https://github.com/Hoverbear/rust-rosetta/issues/100
I've tried the regular expression provided by @Thiez and changed the only cases I found to snake_case per Rust style-guide.

```
$ git grep -Ee 'let [a-z]+[A-Z]|fn [a-z]+[A-Z]' src
src/huffman_coding.rs:        let newNode = HNode{
src/huffman_coding.rs:        let newNode = HNode {
src/huffman_coding.rs:    let rightChar = match right.item {
src/huffman_coding.rs:    let leftChar = match left.item {
src/huffman_coding.rs:    let rightChar = match right.item {
```