Add calldata keyword

Author: Layerex

solidity-mode.el

@@ -95,6 +95,7 @@ Possible values are:
  "assembly"
  "assert"
  "break"
+ "calldata"
  "catch"
  "constant"
  "constructor"
@@ -178,6 +179,7 @@ Possible values are:
  "indexed"
  "storage"
  "memory"
+ "calldata"
  )
  "Modifiers of variables in solidity.")