Use stack allocation in TrSpendInfo #2466

Job	Run time
Lint - nightly toolchain (recent)	45s
Test - nightly toolchain (recent)	11m 34s
Bench - nightly toolchain	3m 43s
Format - nightly toolchain	20s
Docs - nightly toolchain	21s
Integration tests - stable toolchain	6m 4s
Docs - stable toolchain	47s
Test - nightly toolchain (minimal)	11m 0s
Embedded	1m 41s
Test - 1.63.0 toolchain (minimal)	22m 26s
Test - 1.63.0 toolchain (recent)	21m 35s
Test - stable toolchain (recent)	11m 2s
Test - stable toolchain (minimal)	10m 59s
	1h 42m 17s

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