make BuiltinMap immutable; use builder pattern to avoid double-defensive copying both at Transformer and constructors
addressed issues in code review
isolated the internal state of BuiltinMap.Builder completely from effect of outside change; this also simplifies the code
Merge pull request #537 from yilongli/cleanup-BuiltinMap-api

Cleanup builtin map api
added builtin size operation for collections

Conflicts:
	include/builtins/list.k
	include/builtins/map.k
	include/builtins/set.k
Merge branch 'unified-builtins2' of https://github.com/andreistefanescu/k into unified-builtins2