Eval returns the value of the root node of AST, so ensure
that node values are propagated back to ExprStmt and BlockStmt,
which should be sufficient to handle evaluation of simple
statements.
* ast: improve error handling
* ast: handle select
* dyngo: set filename of executed script
* cfg: improve error reporting
* Implement select for receiving channels
* feat(select): add support for sending channels in case clauses
* test: improve tests on select
* feat(select): add support for "default" case