No support for variables in the linker script

[Abestanis]

I have a linker script that contains a variable, and the newest version v0.1.10 fails to parse the linker script because the evaluation logic added in #104 does not take variables into account.

Example linker script with a variable

_stack_guard_size = 0x100;
MEMORY {
    /** ... */
    STACK_GUARD : ORIGIN = 0x20000000, LENGTH = _stack_guard_size
    RAM         : ORIGIN = 0x20000000 + _stack_guard_size, LENGTH = 264K - _stack_guard_size
}

flip-link linker error

error: linking with `flip-link` failed: exit status: 101
  |
  [...]
    = note: thread 'main' panicked at /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/flip-link-0.1.10/src/main.rs:353:13:
          Failed to parse expression "0x20000000 + _stack_guard_size" ("0x20000000 + _stack_guard_size"), got error VariableIdentifierNotFound("_stack_guard_size")
          stack backtrace:
             0: rust_begin_unwind
             1: core::panicking::panic_fmt
             2: flip_link::evaluate_expression
             3: flip_link::main
          note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I used that same linker script with old versions of flip-link and they never complained, but I guess variables were never supported so my RAM sizes must have always been wrong?

I'm not sure manually parsing the entire linker script is a good idea, but it would be possible to use a crate to do the parsing. I've found
ldscript-parser which has not received any activity within a year but seems to be very well tested.

I would be willing to provide a PR trying to replace evalexpr with ldscript-parser if you are on board with this approach.
Just checked and ldscript-parser only supports raw values in the origin and length fields, so we can't use it.