Use atoms as the internal representation of keys in record expressions

[emilsoman]

Currently they are identifiers.

# Record in Fika:
{name: x}

# Is internally represented as
{:record, {1, 0, 9}, nil, [{{:identifier, {1, 0, 5}, :name}, {:identifier, {1, 0, 8}, :x}}]}

# Should be
{:record, {1, 0, 9}, nil, [{:name, {:identifier, {1, 0, 8}, :x}}]}