Approach 8: Use mmap and bytes everywhere (Time: 78s)

In this appraoch we do two things:
1. Use mmap so that we can directly use bytes insted of depending on
   vectors
2. Replace all instances of Vec to bytes

We can see from the previous flamegraph that `to_vec` takes almost 14%
of the cycle in `read_line` function. The motivation to use mmap here
was to convert everything in bytes so that we can reduce the allocation
of memory on Heap for every line.

P.S: I hate the use of unsafe here, but I'm going with it for now and
plan to change it later once I make other stuff performant

Cargo.toml

@@ -14,3 +14,4 @@ rust_decimal = "1.34"
 rust_decimal_macros = "1.34"
 fast-float = "0.2"
 rustc-hash = { version = "1.0"}
+memmap2 = {version = "0.9.4"}