Macro-less variants of expect and expect_file

[jeremyBanks]

Implements the macro-less variants of expect and expect_file using #[track_caller] as described in #15.

The compatibility concern with other functions using #[track_caller] raised in #15 (comment) is noted in the documentation. This isn't entirely a disadvantage; users can write certain wrapper functions with #[track_caller] like they might previously have written wrapper macros:

#[track_caller]
fn assert_debug_eq(expected: impl expect_test::ExpectedData, actual: impl core::fmt::Debug) {
    expect_test::expect(expected).assert_eq(&format!("{actual:?}"));
}

This requires bumping the MSRV, but only by one release (1.45.0 to 1.46.0).