Refactored a bit (added (*environ)evalExprs method).
Added if statements, block statements, and branch statements. Refactored function calls, for statements, and if statements to call block statement code.

(*environ)runStmt method now returns a stmtResult. Return statements, branch statements, and
compound statements that may contain return statements or branch statements might
return a non-nil statement result containing the info returned or the label branched to.

Goto branch statements are not implemented (at least not yet).