refactor!: make use of the mod.rs files instead of <module>.rs files

This helps to organize the code in a better way, and it's more idiomatic
to Rust.

BREAKING CHANGE: this commit changes the way the modules are organized.

benches/array_bench.rs

@@ -1,6 +1,6 @@
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 
-use monkey::{compile_program, execute_interpreter, execute_vm, parse_program};
+use chimpanzee::utils::{compile_program, execute_interpreter, execute_vm, parse_program};
 
 const ARRAY_APPEND: &str = r#"
 let push_n = fn (arr, n) {

benches/fibonacci_bench.rs

@@ -1,6 +1,6 @@
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 
-use monkey::{compile_program, execute_interpreter, execute_vm, parse_program};
+use chimpanzee::utils::{compile_program, execute_interpreter, execute_vm, parse_program};
 
 const FIBONACCI_20: &str = r#"
         let fibonacci = fn(x) {

benches/formatter_bench.rs

@@ -1,6 +1,6 @@
+use chimpanzee::formatter::Formatter;
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 
-use formatter::formatter::Formatter;
 
 pub fn long_formatter_benchmark(c: &mut Criterion) {
     let input = r#"

src/bin/monkey.rs

@@ -1,5 +1,5 @@
 use clap::Parser;
-use chimpanzee::repl::repl::ReplCli;
+use chimpanzee::repl::ReplCli;
 use std::error::Error;
 
 fn main() -> Result<(), Box<dyn Error>> {