Adds `triu` and `tril` methods directly to ArrayBase #128

Job	Run time
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
cargo-careful	0s
docs/${{ matrix.rust }}	0s
clippy/beta	18s
format/nightly	10s
tests/stable	3m 47s
tests/beta	3m 46s
tests/nightly	4m 20s
tests/1.57.0	5m 1s
conclusion	1s
	17m 23s

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