Support for branch/compilation hints

[titzer]

As discussed in this issue, we'd like to support branch and/or compilation hints for new functions. We might also want to support arbitrary other custom sections to be included in the code.

Here's one idea:

Reserve a zero byte in the encoding of func.new and allocate a bit as a flag to indicate that custom sections precede the code. If the flag is set (e.g. 0x1), then the bytecode buffer supplied in memory consists of a list of custom sections, one after another, ending with the code section byte and then the bytecode of the new function.

The custom sections would be encoded similarly to branch/compilation hints now, but would apply to only the new function.