Add missing operations (part 2)

[greenhat]

Leftovers from #324 that are not covered in existing issues.

SBoolean:

• toByte

SNumeric:

• abs
• compare

SBigInt:

• modQ; (soft fork)
• plusModQ; (soft fork)
• minusModQ; (soft fork)
• multModQ; (soft fork)

SGroupElement:

• isIdentity; (soft fork) // special.sigma.GroupElement.isInfinity should be renamed

SOption:

• isEmpty (add rule to OptionCoster); (soft fork)
• toColl (add rule to OptionCoster); (soft fork)
• flatMap (add rule to OptionCoster); (soft fork)
• fold (add rule to OptionCoster); (soft fork)

SCollection: (all via soft fork)

• flatMap;
• find;
• mapReduce;
• groupBy;
• groupByProjecting;
• unionSets;
• diff;
• intersect;
• sum;
• prefixLength;
• lastIndexOf;
• distinct;
• startsWith;
• endsWith;

[aslesarenko]

[by kushti in Discord]
if we're going to rename "isInfinity" to "isIdentity" , it makes sense to rename "negate" to "inverse" as well (negate is inverse in additive notation, and we're using multiplicative)

[kushti]

Collection and Numeric methods are moved to #1004 and #1006 , correspondingly

SBigInt methods are not actual, will be replaced with new datatype from #997

SOption.empty already done in the frontend compiler. Other Soption methods can be done with one-liners

SBoolean.toByte can be done with one-liner

isIdentity can be done by simple comparison with encoded identity value (do a test for that and then the issue can be closed)