feat: alt constructor entrypoint

[Maddiaa0]

Overview

Similarly to #221, adds a command line flag that allows for compiling an alternative constructor.

Usage

The following command
huffc ./test.huff -b -l ALT_CONSTRUCTOR
test file yields:

#define macro CONSTRUCTOR() = {
    0x1
}

#define macro MAIN() = {
    0x1
}

#define macro ALT_CONSTRUCTOR() = {
    0x2 0x00 0x00 0x00
}

600260006000600060028060113d393df36001, which is as expected.

RFC:

• Currently -c is already taken so I have decided to use -l as the short arg. Do you have a preference?

closes #222

[Maddiaa0]

@abigger87 Looks like new string interpolation syntax is triggering clippy. Reckon this pr is fine as is and the clippy warnings should be fixed in the seperate pr?

edit:
clippy fix pr: #224

[refcell]

LGTM