chore(Cargo.toml): Better document direct deps

- Slight adjustments to the version fields for compatibility with
  `cargo +nightly update -Z direct-minimal-versions` for MSRV `1.56.0`
- Add `rust-version` field for leveraging 
  `cargo +nightly update -Z msrv-policy` to generate a lockfile that
  respects the MSRV, and the benefit of downstreams.
- Communicate why `dev-dependencies` are required (examples / tests).
- Avoid repeating deps in `dev-dependencies`.
- Raise fixed `warp` dev dep to a MSRV compatible version with common
  `tokio-util` implicit dep. Simplifies CI lock maintenance.

Signed-off-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

Cargo.toml

@@ -10,6 +10,7 @@ authors = ["Ryan Leckey <leckey.ryan@gmail.com>"]
 categories = ["config"]
 license = "MIT/Apache-2.0"
 edition = "2018"
+rust-version = "1.56.0"
 
 [badges]
 maintenance = { status = "actively-developed" }
@@ -23,13 +24,13 @@ json5 = ["json5_rs"]
 preserve_order = ["indexmap", "toml/preserve_order", "serde_json/preserve_order", "ron/indexmap"]
 
 [dependencies]
-async-trait = "0.1.50"
-lazy_static = "1.0"
-serde = "1.0.8"
+async-trait = "0.1.2"
+lazy_static = "1.4"
+serde = "1.0.99"
 nom = "7"
 
 toml = { version = "0.5", optional = true }
-serde_json = { version = "1.0.2", optional = true }
+serde_json = { version = "1.0.0", optional = true }
 yaml-rust = { version = "0.4", optional = true }
 rust-ini = { version = "0.18", optional = true }
 ron = { version = "0.7", optional = true }
@@ -38,16 +39,24 @@ indexmap = { version = "1.7.0", features = ["serde-1"], optional = true}
 pathdiff = "0.2"
 
 [dev-dependencies]
-serde_derive = "1.0.8"
 float-cmp = "0.9"
-chrono = { version = "0.4", features = ["serde"] }
-tokio = { version = "1", features = ["rt-multi-thread", "macros", "fs", "io-util", "time"]}
-warp = "=0.3.1"
-futures = "0.3.15"
-reqwest = "0.11.10"
 
-serde = "1.0"
+# tests/legacy (available in serde via `features = ["derive"]` since 1.0.103)
+serde_derive = "1.0.99"
+
+# tests/datetime.rs + tests/legacy/datetime.rs
+chrono = { version = "0.4.23", features = ["serde"] }
+
+# examples/async_source
+warp = "=0.3.5"
+futures = "0.3"
+reqwest = "0.11"
+# Additionally tests/async_builder.rs
+tokio = { version = "1.13", features = ["rt-multi-thread", "macros"]}
+
+# examples/glob
 glob = "0.3"
-lazy_static = "1"
-notify = "^4.0.0"
+# examples/watch
+notify = "4.0.0"
+# tests/env.rs
 temp-env = "0.2.0"