MSRV should be 1.40

[lemmih]

Hi,

Line 109 in src/lib.rs uses cfg(doctest) which didn't stabilize until Rust version 1.40.

[fogti]

If the MSRV is bumped, the following work-around could be dropped as well:

walkdir/src/dent.rs

Lines 179 to 191 in abf3a15

	/// Returns true if and only if this entry points to a directory.
	///
	/// This works around a bug in Rust's standard library:
	/// https://github.com/rust-lang/rust/issues/46484
	#[cfg(windows)]
	pub(crate) fn is_dir(&self) -> bool {
	use std::os::windows::fs::MetadataExt;
	use winapi::um::winnt::FILE_ATTRIBUTE_DIRECTORY;
	self.metadata.file_attributes() & FILE_ATTRIBUTE_DIRECTORY != 0
	}
	/// Returns true if and only if this entry points to a directory.
	#[cfg(not(windows))]