Cairo formatting

[ericglau]

Fixes #216

This follows the general Contracts for Cairo format, with slight differences for more consistency between different functions.

• Keeps implicit args on the same line as the function name
• If there are args, use a separate line for args, then another line for return types e.g.

func balanceOf{syscall_ptr: felt*, pedersen_ptr: HashBuiltin*, range_check_ptr}(
    account: felt, id: Uint256
) -> (balance: Uint256) {

• If args line is too long, wraps it as one arg per line:

func safeBatchTransferFrom{syscall_ptr: felt*, pedersen_ptr: HashBuiltin*, range_check_ptr}(
    from_: felt,
    to: felt,
    ids_len: felt,
    ids: Uint256*,
    values_len: felt,
    values: Uint256*,
    data_len: felt,
    data: felt*,
) {

• If there are no args, keep return type on the same line as function name e.g.

func name{syscall_ptr: felt*, pedersen_ptr: HashBuiltin*, range_check_ptr}() -> (name: felt) {

[frangio]

Looks good!