various build warnings

[Michael Madden]

$ rustc --version
rustc 1.52.1 (9bc8c42bb 2021-05-09)

$ sw_vers
ProductName:    macOS
ProductVersion: 11.3.1
BuildVersion:   20E241

I see this warning from cargo build --release:

warning: variable does not need to be mutable
   --> src/render.rs:160:13
    |
160 |         let mut t = app
    |             ----^
    |             |
    |             help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` on by default

warning: associated function is never used: `new`
    --> src/render.rs:1319:12
     |
1319 |     pub fn new() -> SectionMGRList<'a> {
     |            ^^^
     |
     = note: `#[warn(dead_code)]` on by default

warning: 2 warnings emitted