Add a new interface: DeviceConfig::from_env()

[sukyoungjeong-furiosa]

fixes #59 depends on #61

Major Changes

• DeviceConfig now parses comma-separated syntaxes (e.g., 0:0,0:1 or warboy(1)*2,warboy(2)*1 ..)
• DeviceConfig now supports npu0pe0-like textual representations
• Introduce DeviceConfig::from_env() and related EnvBuilder
  • One can use it like DeviceConfig::from_env("FURIOSA_DEVICES").or_env("NPU_DEVNAME").or_default().build()?
• Add documentations for DeviceConfig's textual representation

Minor Changes

• Add eyre as dev-dependency for better test cases
• Update toolchain to nightly
• Add rustfmt.toml