Rename parameter that shadows built-in type

The parameter `char` in `insert_char` shadows the built-in `char` type.

**Location:** [basalt/src/input/mod.rs#L150](https://github.com/erikjuhani/basalt/blob/5e628ef/basalt/src/input/mod.rs#L150)

**Change:** Rename the parameter to `c` or `character`:
```rust
pub fn insert_char(&mut self, c: char)
```
