./x.py clippy: allow the most noisy lints
./x.py clippy: allow the most noisy lints

This silences the following clippy lints in ./x.py clippy:

many_single_char_names (there are a lot of warnings caused by stdarch)
collapsible_if (can reduce readability)
type_complexity
missing_safety_doc (there are almost 3K warnings issued)
too_many_arguments
needless_lifetimes (people want 'tcx lifetimes etc)
wrong_self_convention (warns about from_..(), to_..(), into_..().. fns that do or do not take self by reference.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust