Implement missing import formats

[spalladino]

Taken from solidity-parser-antlr:

importDirective
  : 'import' StringLiteral ('as' identifier)? ';'
  | 'import' ('*' | identifier) ('as' identifier)? 'from' StringLiteral ';'
  | 'import' '{' importDeclaration ( ',' importDeclaration )* '}' 'from' StringLiteral ';' ;