Populate structs from cli args and/or .env file

Intended to be used like this:

```rs, norun use fromenv::FromEnv; use lazystatic::lazystatic; use serde::Deserialize;

fn cred_file() -> String { "credentials.json".into() }

fn server_url() -> String { "127.0.0.1:8080".into() }

[derive(Debug, Clone, Deserialize)]

pub struct Constants { #[serde(default = "credfile")] pub credfile: String, #[serde(default = "serverurl")] pub serverurl: String, }

lazystatic! { pub static ref CONSTANTS: Constants = Constants::fromenv().expect("Please provide valid args for constants"); } ```

Now you can either provide values for cred_file and server_url via CLI or .env file, or a mix of both. Any value can be left out. CLI values override .env files, which in turn override defaults.

with a .env file:

txt cred_file = credentials.json

or directly in the CLI:

txt cargo run -- --server_url localhost://8080

It uses serde_json under the hood.