BREAKING fix macro string parsing

[aep]

string is a vec due to different bytestring interpretations, so ast has to parse this as vec.

While we're at it, this is a breaking change to move ast to a dynamically allocated tree, preparing for struct macros (derive).

refer to tests/mustpass/macro_expr for how the api changed

[aep]

@jwerle breaking change to macros incomming

[jwerle]

Checking this out now