Fix clippy error: slow zero-filling initialization

[obbardc]

Clippy produces an error:

error: slow zero-filling initialization
Error:   --> bmap-parser/src/lib.rs:62:5
   |
60 |     let mut v = Vec::new();
   |                 ---------- help: consider replacing this with: `vec![0; 8 * 1024 * 1024]`
61 |     // TODO benchmark a reasonable size for this
62 |     v.resize(8 * 1024 * 1024, 0);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: slow zero-filling initialization
Error:    --> bmap-parser/src/lib.rs:109:5
    |
107 |     let mut v = Vec::new();
    |                 ---------- help: consider replacing this with: `vec![0; 8 * 1024 * 1024]`
108 |     // TODO benchmark a reasonable size for this
109 |     v.resize(8 * 1024 * 1024, 0);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Fix it by following the suggestions.