Implement New Type and Symbol Systems

[LPTK]

To make the new IR faster, and compatible with ScalaJS/ScalaNative (and also to lower its startup time), we would like not to rely on Scala Reflection at runtime.

This means we implement our own representation of types, type symbols and method symbols.

The downside is that we won't be able to inspect at runtime the annotations (such as effects annotations) defined on method symbols. To work around this, we:

• will provide nice tools to provide effects annotations manually for external libraries (in the same vein as this);
• will extend the functionalities of the @embed macro annotation to look at the annotations placed on methods and register them in the IR;
• also, we have to remove the runtime dependencies on Scala Reflection that currently exists in the code that @embed generates.