fix solidity warnings in smart contract template

[lovesh]

There are warning in code like

1. Variable is shadowed in inline assembly by an instruction of the same name, this is because Pairing defines functions with names add and mul which conflict assembly.
2. Function state mutability can be restricted to pure, those functions should be made pure.
3. Invoking events without "emit" prefix is deprecated., logging events should be done by using emit
4. The use of non-functional instructions is deprecated. Please use functional notation instead., opcode invalid should be used as invalid()

Signed-off-by: Lovesh Harchandani lovesh.bond@gmail.com

[JacobEberhardt]

This is nice, thank you