Inline fixed-width PUSH immediates

[Giulio2002]

Summary

• generate fixed-width inline cases for PUSH5-PUSH19 and PUSH21-PUSH31
• avoid the generic uint256.Int.SetBytes path for these static immediate widths
• keep existing specialized PUSH0-PUSH4, PUSH20, and PUSH32 cases

Benchmark

Focused BenchmarkOpcode PUSH cases reproduced on this branch:

• PUSH1_POP: unchanged baseline/control
• PUSH5_POP: around 6.6ms/op
• PUSH16_POP: around 6.6ms/op
• PUSH31_POP: around 9.1ms/op

Previous baseline comparison for this exact change showed:

• Opcode/PUSH5_POP: 9.908ms -> 6.657ms, -32.82%
• Opcode/PUSH16_POP: 9.806ms -> 6.659ms, -32.09%
• Opcode/PUSH31_POP: 10.193ms -> 8.718ms, -14.47%

Verification

• go generate ./vm
• git diff --check
• go test ./vm -count=1
• go test ./... -count=1
• GEVM_TEST_RUNNER=opcode go test ./... -count=1
• go test ./tests/bench -run '^$' -bench '^BenchmarkOpcode/(PUSH1_POP|PUSH5_POP|PUSH16_POP|PUSH31_POP)$' -benchmem -count=6

[Giulio2002]

@copilot can you merge origin/main in?