support ghost definitions

[pickx]

ghost definitions have a different syntax that would not be recognized by the current parser:

ghost $NAME($TYPE_LIST) $RETURNS_TYPE_OR_AXIOM_LIST
where

• $TYPE_LIST is a (possibly empty) series of types separated by a comma.
• $RETURNS_TYPE_AXIOM_LIST is either returns $TYPE; or some text delimited by curly brackets.

to make things worse, ghost also supports GhostMapDecl of the form ghost mapping($TYPE => $TYPE) $NAME $RETURNS_TYPE_OR_AXIOM_LIST.

examples from RealLifeCVTApplications:

• ghost toAddress(address) returns ADDRESS;
• ghost shadowSnapshots(ADDRESS,COUNTER) returns uint256;
• ghost assetListLen() returns uint { //... }
• ghost mapping(address => mapping(uint => uint)) multiDim2;
• ghost mapping(uint256 => mathint) sumWithdrawalsPerStream { //... }

[pickx]

fixed by #8