Add basic block runner mode

[Giulio2002]

Summary

• add a generated basic-block default runner while keeping the opcode tracing runner available
• add benchmarks comparing default and tracing runner paths
• pin and download spec fixtures for make test
• allow spec tests to run through the opcode/tracing path with GEVM_TEST_RUNNER=opcode

Verification

• make test
• GEVM_TEST_RUNNER=opcode make test
• go test ./... -count=1
• GEVM_TEST_RUNNER=opcode go test ./... -count=1