New lint: suggest using fs::{read,read_to_string} over File counterparts

[tesuji]

In specific, we recommend to use:

• std::fs::read_to_string over File::read_to_string
• std::fs::read over File::read_to_end

But only when buffer is empty.

[flip1995]

https://doc.rust-lang.org/std/fs/fn.read.html

This is a convenience function for using File::open and read_to_end with fewer imports and without an intermediate variable.

https://doc.rust-lang.org/std/fs/fn.read_to_string.html

This is a convenience function for using File::open and read_to_string with fewer imports and without an intermediate variable.

[jmeyers35]

Hi, could I work on this? I'd like to get started contributing to Clippy. Thanks!

[flip1995]

Yeah sure! If you have any questions, just ask here or open a WIP PR. A good read to get started is this document: https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md