Certain attributes not represented in grammar

[mira-alford]

There are certain attributes in the AST right now that do not exist in the grammar and cannot survive a round trip (they get printed, but will then fail to parse). This only comes up when the attributes are added from code, so it's hard to produce an example, however the memory encoding currently does generate such an example as of #161 inserting boogie attributes into ApplyFun expressions. This gets added to free but does not parse:

requires ($me_addr_is_heap) { .boogie = { .msg = "\"Memory Error: Invalid Free\"" } } ($mem_encoding, R0_in:bv64)

It seems like all of the expression variants have unused attribtues: ApplyFun, Binary, Lambda, Unary, Let and ApplyIntrin. Most declarations are fine but variable declaration also have this issue.

[agle]

The output is probably ambiguous, it might make sense to just attach an attrib to the parenthesized expression case and make the output always emit parenths when attribs are present so its clear what they refer to

ExprParen = "(" expr1 ")";
ExprParenAttr = "(" attribset expr1 ")";

Except for lambda exprs where we've placed them similar ish to boogie.