o1vm/mips: use biguint instead of Fp in witness builder

[dannywillems]

Not getting better perfs, we should only use u64 and have a special state element in the env for MIPS_PREIMAGE_KEY with BigUint.

[2024-11-25T16:44:24Z INFO  o1vm::interpreters::mips::witness] processing step=11000000 pc=0000134b24 insn=008fa20078 ips=229166.67 pages=8748 mem=34.2 MiB name=reflect.(*rtype).Field
[2024-11-25T16:44:28Z INFO  o1vm::interpreters::mips::witness] processing step=12000000 pc=0000213ce4 insn=0015800020 ips=230769.23 pages=8759 mem=34.2 MiB name=regexp/syntax.(*parser).parseClass
[2024-11-25T16:44:32Z INFO  o1vm::interpreters::mips::witness] processing step=13000000 pc=000021462c insn=001120004b ips=236363.64 pages=8779 mem=34.3 MiB name=regexp/syntax.appendRange
[2024-11-25T16:44:35Z INFO  o1vm::interpreters::mips::witness] processing step=14000000 pc=000002cc5c insn=002c270200 ips=237288.14 pages=8794 mem=34.4 MiB name=runtime.writeHeapBits.flush
[2024-11-25T16:44:39Z INFO  o1vm::interpreters::mips::witness] processing step=15000000 pc=00000ae49c insn=00ac2efff8 ips=241935.48 pages=8805 mem=34.4 MiB name=runtime.memmove
[2024-11-25T16:44:42Z INFO  o1vm::interpreters::mips::witness] processing step=16000000 pc=00001d4000 insn=0024860001 ips=242424.24 pages=8815 mem=34.4 MiB name=encoding/json.checkValid
[2024-11-25T16:44:46Z INFO  o1vm::interpreters::mips::witness] processing step=17000000 pc=000002ca2c insn=00afa00034 ips=246376.81 pages=8820 mem=34.5 MiB name=runtime.writeHeapBits.write
[2024-11-25T16:44:49Z INFO  o1vm::interpreters::mips::witness] processing step=18000000 pc=000002247c insn=0071ce7020 ips=246575.34 pages=8824 mem=34.5 MiB name=runtime.mallocgc
[2024-11-25T16:44:52Z INFO  o1vm::interpreters::mips::witness] processing step=19000000 pc=000002cc4c insn=008ee6bd14 ips=250000.00 pages=8830 mem=34.5 MiB name=runtime.writeHeapBits.flush
[2024-11-25T16:44:56Z INFO  o1vm::interpreters::mips::witness] processing step=20000000 pc=00003391f8 insn=008c870080 ips=253164.56 pages=8836 mem=34.5 MiB name=gopkg.in/yaml%2ev3.yaml_parser_update_buffer
[2024-11-25T16:45:00Z INFO  o1vm::interpreters::mips::witness] Snapshot state in snapshot-state-20000000.json, step 20000000
[2024-11-25T16:45:03Z INFO  o1vm::interpreters::mips::witness] processing step=21000000 pc=000033c98c insn=0030a60080 ips=241379.31 pages=8842 mem=34.5 MiB name=gopkg.in/yaml%2ev3.read
[2024-11-25T16:45:07Z INFO  o1vm::interpreters::mips::witness] processing step=22000000 pc=0000338f50 insn=0000414823 ips=244444.44 pages=8851 mem=34.6 MiB name=gopkg.in/yaml%2ev3.yaml_parser_update_buffer
[2024-11-25T16:45:10Z INFO  o1vm::interpreters::mips::witness] processing step=23000000 pc=000033d770 insn=00afa30068 ips=244680.85 pages=8854 mem=34.6 MiB name=gopkg.in/yaml%2ev3.yaml_parser_fetch_next_token
[2024-11-25T16:45:14Z INFO  o1vm::interpreters::mips::witness] processing step=24000000 pc=0000349c24 insn=00afa80044 ips=247422.68 pages=8863 mem=34.6 MiB name=gopkg.in/yaml%2ev3.yaml_parser_scan_plain_scalar
[2024-11-25T16:45:17Z INFO  o1vm::interpreters::mips::witness] processing step=25000000 pc=00001c87a4 insn=0000000000 ips=247524.75 pages=8867 mem=34.6 MiB name=encoding/json.unquoteBytes
[2024-11-25T16:45:20Z INFO  o1vm::interpreters::mips::witness] processing step=26000000 pc=0000349730 insn=0000011825 ips=250000.00 pages=8872 mem=34.7 MiB name=gopkg.in/yaml%2ev3.yaml_parser_scan_plain_scalar
[2024-11-25T16:45:24Z INFO  o1vm::interpreters::mips::witness] processing step=27000000 pc=0000349640 insn=0024420004 ips=252336.45 pages=8878 mem=34.7 MiB name=gopkg.in/yaml%2ev3.yaml_parser_scan_plain_scalar
[2024-11-25T16:45:27Z INFO  o1vm::interpreters::mips::witness] processing step=28000000 pc=0000148a3c insn=0030220060 ips=252252.25 pages=8881 mem=34.7 MiB name=reflect.valueInterface
[2024-11-25T16:45:31Z INFO  o1vm::interpreters::mips::witness] processing step=29000000 pc=00000610ac insn=00ac240014 ips=254385.96 pages=8886 mem=34.7 MiB name=runtime.deferreturn
[2024-11-25T16:45:34Z INFO  o1vm::interpreters::mips::witness] processing step=30000000 pc=000021f3bc insn=00240e0002 ips=254237.29 pages=8891 mem=34.7 MiB name=regexp.(*Regexp).doOnePass