Self-check failures on 32-bit i386 systems

Hi. There appears to be some 64-bitisms in the code. Could you support 32-bit targets?

https://salsa.debian.org/jas/sigsum-go/-/jobs/5178882

    client_test.go:171: bad signature: (expected) failure: monitoring alert: Invalid log signature: log signature invalid
    client_test.go:171: bad signature (hash): (expected) failure: monitoring alert: Invalid log signature: log signature invalid
    client_test.go:171: bad consistency: (expected) failure: monitoring alert: Log tree head not consistent: consistency proof not valid: invalid proof: old root mismatch
...
ok  	sigsum.org/sigsum-go/pkg/submit	0.063s
# sigsum.org/sigsum-go/pkg/types [sigsum.org/sigsum-go/pkg/types.test]
src/sigsum.org/sigsum-go/pkg/types/tree_head_test.go:235:4: cannot use testCosignTimestamp (untyped int constant 72623859790382856) as int value in argument to t.Errorf (overflows)
?   	sigsum.org/sigsum-go/tests/mk-add-tree-request	[no test files]