Adds `triu` and `tril` methods directly to ArrayBase #126

Job	Run time
clippy/beta	20s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
docs/${{ matrix.rust }}	0s
cargo-careful	0s
format/nightly	13s
tests/stable	3m 59s
tests/beta	3m 47s
tests/nightly	3m 57s
tests/1.57.0	7m 16s
conclusion	1s
	19m 33s

Provide feedback