Non-constant type asserts are not yet implemented, but should be
relatively easy to add at a later time. They should result in a clear
error message for now.
This implements the copy() built-in function. It may not work in all
cases, but should work in most cases.
This commit gets the following 3 packages to compile, according to
tinygo-site/imports/main.go:
* encoding/base32
* encoding/base64
* encoding/pem (was blocked by encoding/base64)
