Improve batched serial trsv (#2452)

* Add missing specialization of Serial Trsv

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

* Improve testing of serial trsv

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

* format

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

* fix code QR

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

* simplify tests for serial trsv

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

* format

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

* static-assertion check outside debug mode

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

* disallow vector_length=8 if AVX512 is unavailable

Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>

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Signed-off-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>
Co-authored-by: Yuuichi Asahi <y.asahi@nr.titech.ac.jp>