Adds `triu` and `tril` methods directly to ArrayBase #104

Job	Run time
cargo-careful	0s
clippy/beta	17s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
docs/${{ matrix.rust }}	0s
format/nightly	10s
tests/stable	26s
tests/beta	1m 1s
tests/nightly	39s
tests/1.57.0	31s
conclusion	1s
	3m 5s

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