Exact version dependency on TOML causes version resolution conflicts

[LPGhatguy]

Hello!

In 703b673a0d41034a457cc56095f1b51d884953d5, the toml crate was changed to an exact dependency on version 0.5.8. Based on the appveyor link, it looks like this was done because of toml using a feature stabilized in Rust 1.36.0 (impl From<&String> for String)[^1], but embed-resource's MSRV is 1.35.0.

This change prohibits users of embed-resource from also using toml 0.5.9. In my project's case, that release includes a fix for a specific bug that produces invalid TOML.

When resolving dependencies, Cargo does not allow two semver-compatible versions of the same crate to co-exist, even when one is a build dependency. As a result, libraries should never use exact version dependencies. They often result in version resolution conflicts and break some built-in Cargo commands, like cargo add:

$ cargo add toml
    Updating crates.io index
      Adding toml v0.5.9 to dependencies.
             Features:
             - indexmap
             - preserve_order
error: failed to select a version for `toml`.
    ... required by package `embed-resource v1.7.4`
    ... which satisfies dependency `embed-resource = "^1.7.4"` of package `cyrtanthus v0.1.0`
    ... which satisfies path dependency `cyrtanthus` (locked to 0.1.0) of package `cyrtanthus-desktop v0.1.0`
versions that meet the requirements `=0.5.8` are: 0.5.8

all possible versions conflict with previously selected packages.

  previously selected package `toml v0.5.9`
    ... which satisfies dependency `toml = "^0.5.9"` of package `cyrtanthus v0.1.0)`
    ... which satisfies path dependency `cyrtanthus` (locked to 0.1.0) of package `cyrtanthus-desktop v0.1.0)`

failed to select a version for `toml` which could resolve this conflict

I am aware of a couple options that could fix this issue:

1. Raise embed-resource's MSRV to at least 1.36.0 (released July 2019) and remove this exact version dependency.
2. File an issue with the toml authors to push a new 0.5.10 release that supports Rust 1.35.0. This seems unlikely: 1.36.0 is a very common MSRV and it appears the authors are in the middle of a major refactor for toml 0.6.

[^1]: Rust's stable docs indicate that this was stabilized in Rust 1.35.0. However, the earliest version of Rust that I could find this implementation in is Rust 1.36.0: https://doc.rust-lang.org/1.36.0/std/string/struct.String.html#impl-From%3C%26%27_%20String%3E

[nabijaczleweli]

There's recent precedent for this in cargo-update 11.0.0, and the same applies here unfortunately – relaxed to "0.5".

[nabijaczleweli]

Released in v1.8.0