Rename Arrow to DoubleArrow

docs/grammar/Solidity.g4

@@ -433,7 +433,7 @@ variableDeclarationTuple:
 variableDeclarationStatement: ((variableDeclaration (Assign expression)?) | (variableDeclarationTuple Assign expression)) Semicolon;
 expressionStatement: expression Semicolon;
 
-mappingType: Mapping LParen key=mappingKeyType Arrow value=typeName RParen;
+mappingType: Mapping LParen key=mappingKeyType DoubleArrow value=typeName RParen;
 /**
  * Only elementary types or user defined types are viable as mapping keys.
  */

docs/grammar/SolidityLexer.g4

@@ -111,7 +111,7 @@ Colon: ':';
 Semicolon: ';';
 Period: '.';
 Conditional: '?';
-Arrow: '=>';
+DoubleArrow: '=>';
 RightArrow: '->';
 
 Assign: '=';