Adds `triu` and `tril` methods directly to ArrayBase #122

Job	Run time
docs/${{ matrix.rust }}	0s
cargo-careful	0s
clippy/beta	17s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
format/nightly	12s
tests/stable	30s
tests/beta	33s
tests/nightly	27s
tests/1.57.0	44s
conclusion	1s
	2m 44s

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