[Stdlib] Add type check macros

[alexmozaidze]

Having simple macros like string?, number?, table?, nil?, etc. would increase the readability of code that may use them.

The only thing I'm worried about is the existing table? in macro context. iirc, it checks if an AST node is a table literal, as opposed to runtime type of table. I think it wouldn't be unreasonable to rename AST checking macros to be: table-literal?, sequence-literal?, etc.

[technomancy]

The big problem here is that it will almost certainly conflict with existing identifiers in a large number of existing codebases. Until we have a mechanism to opt in to new built-in macros, I don't think we can introduce these as it will be a breaking change.