Configure clippy to error on warnings

[charlesbluca]

Adds -D warnings to the clippy entrypoint in our pre-commit config so that warnings will now cause our linting to fail. Will also attempt to fix the existing warnings in this PR.

[charlesbluca]

Just want to note that we can also configure clippy to automatically fix some of the more easily corrected warnings with

Suggested change

	args: ['--manifest-path', './dask_planner/Cargo.toml', '--verbose', '--', '-D', 'warnings']
	args: ['--manifest-path', './dask_planner/Cargo.toml', '--fix', '--allow-dirty', '--verbose', '--', '-D', 'warnings']

Was just hesitant to do this by default as the error message when leaving out --allow-dirty seemed to imply that this could result in destructive changes; @andygrove @jdye64 do you have any preferences here?

[andygrove]

I would prefer that we fix any warnings ourselves, even if we are often just copying & pasting Clippy's suggestions. I think this is an excellent way to learn more about writing idiomatic Rust code.

[jdye64]

Agree with Andy's comments here

[jdye64]

For clarity is the plan to manually fix all the clippy warnings in this PR or a subsequent one?

[charlesbluca]

@jdye64 plan is to manually fix all the clippy warnings in this PR, I've managed to narrow it down to only two:

error: deref on an immutable reference
   --> src/sql/types.rs:272:36
    |
272 |     pub fn from_string(input_type: &str) -> Self {
    |                                    ^ help: if you would like to reborrow, try removing `&*`: `&`
    |
    = note: `-D clippy::borrow-deref-ref` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_deref_ref

error: deref on an immutable reference
   --> src/sql.rs:229:34
    |
229 |     pub fn parse_sql(&self, sql: &str) -> PyResult<Vec<statement::PyStatement>> {
    |                                  ^ help: if you would like to reborrow, try removing `&*`: `&`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_deref_ref

error: could not compile `dask_planner` due to 2 previous errors

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[jdye64]

LGTM