MyriaL support for SplitExpression and SequenceExpression

[senderista]

These are currently the only built-in flatmap expressions in MyriaX, so can be supported in RACO without syntax support for MyriaL flatmap UDFs. Tested in MyriaWeb with the following queries:

R = load("https://s3-us-west-2.amazonaws.com/uwdb/sampleData/tab.txt",
csv(schema(index:int, intlist:string), delimiter="\t"));
store(R, flatmap_input, [index, intlist]);
S = scan("public:adhoc:flatmap_input");
T = [from S emit S.index, S.intlist, split(S.intlist, ",")];
store(T, flatmap_output, [$0, $1, $2]);
T = [from S emit S.index, S.intlist, sequence(S.index)];
store(T, sequence_output, [$0, $1, $2]);

[coveralls]

Coverage decreased (-0.05%) to 91.829% when pulling 61340fc on flatmap into 0231c09 on master.

[shrjain]

LGTM

[coveralls]

Coverage decreased (-0.05%) to 91.829% when pulling 61340fc on flatmap into 0231c09 on master.

[coveralls]

Coverage decreased (-0.1%) to 91.756% when pulling 335c8ca on flatmap into 0231c09 on master.