Allow non-numeric function identifiers

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Format of Function Identifiers

This proposal requires that function identifiers be stack items of length 0 to 7, inclusive. This range maximizes flexibility while ensuring that VM implementations can rely on a single 64-bit key for function table implementations (including both the identifier's length and value).

Integer-based function identifiers are particularly appropriate given the existing design of the VM: specialized, number pushing opcodes occupy 18 codepoints in the instruction set (OP_1NEGATE and OP_0 to OP_16), enabling up to 18 unique function identifiers to be encoded using a single byte. However, function identifiers are not required to be minimally encoded numbers, avoiding numeric decoding overhead and enabling a wider variety of usage patterns – e.g. "namespaces" (like 0x00 through 0xff), the single-byte OP_1NEGATE, and other identifiers which cannot be parsed as minimal VM numbers.

Outperforming this proposal's approach (in terms of byte-efficiency per encoded invocation) would require reserving a range of user-definable OP_INVOKE0 to OP_INVOKE16 (saving 1 byte per invocation). However, function definitions beyond that range would again require use of a generic OP_INVOKE (or generic multi-byte opcode range to specify through e.g. OP_INVOKE999). As single-byte codepoints are very limited, this proposal considers the two-codepoint OP_DEFINE/OP_INVOKE approach to be most prudent.