Implement EIP-7979 CALLSUB, CALLDEST, RETURNSUB (EVMC_EXPERIMENTAL)

[gcolvin]

Draft. Implements EIP-7979 (proposed for Hegotá) in the Baseline interpreter, gated on EVMC_EXPERIMENTAL, so that compilers targeting the EIP have an EVM to test against. Solidity's IR pipeline can already emit these instructions under --evm-version @future (argotorg/solidity#17005), and its CI runs on evmone: with this change those tests can execute.

The instructions

	opcode	gas	semantics
CALLSUB	0xB0	8	pop destination (must be a CALLDEST), push next position to the return stack, jump
CALLDEST	0xB1	1	subroutine entry; no-op like JUMPDEST; also a valid JUMP/JUMPI destination
RETURNSUB	0xB2	5	pop the return stack into the program counter

Return stack limit 1024 per frame. Failures: invalid destination → EVMC_BAD_JUMP_DESTINATION; return-stack overflow / underflow → EVMC_STACK_OVERFLOW / EVMC_STACK_UNDERFLOW.

Design notes

• The code analysis computes a second bitset of CALLDEST positions in the same pass as the JUMPDEST one, sharing the same allocation.
• Because CodeAnalysis is cached per code independently of revision, CALLDESTs are kept out of the jumpdest bitset. jump_impl consults the CALLDEST bitset only when the JUMPDEST test fails and the revision has EIP-7979, so ordinary jumps are unchanged and behaviour before the EIP is byte-for-byte what it was (a jump to a 0xB1 byte is still a bad jump destination).
• The return stack is a std::vector<uint32_t> on ExecutionState, cleared by reset(), so it is per frame by construction.
• The Advanced interpreter maps the three opcodes to op_undefined, as EIP-8024's instructions are.

Tests

evm_eip7979_callsub_test.cpp: the EIP's five test cases with their gas totals (17, 34, 29), invalid destinations (JUMPDEST, PUSH data, out of range, uint64 overflow, stack underflow), JUMP/JUMPI landing on a CALLDEST, RETURNSUB after an unframed jump, tail call, CALLDEST by fall-through, a value returned through a subroutine, the 1024 limit from both sides (1023-deep recursion succeeds, 1024 halts), return-stack reset between executions, and pre-EIP behaviour (undefined instruction, 0xB1 not a jump destination). All 1285 unit tests pass.

End to end

The demo contract from the Solidity PR, compiled with subroutines and run on this build with the mocked host:

compute(a, b)	legacy build, Osaka	subroutine build, Amsterdam	subroutine build, EVMC_EXPERIMENTAL
(3, 4)	49, 1008 gas	undefined instruction	49, 889 gas
(0, 0)	1, 504 gas	undefined instruction	1, 454 gas
(10, 5)	245, 1176 gas	undefined instruction	245, 1034 gas