Add Tree.of and Term.of to transform Expr into Tree

[nicolasstucki]

This is replacement for Expr.asReflectTree (which replaced unseal). This version of the API aligns with Type.of, TypeRepr.of and TypeTree.of. By requiring the Tree or Term modules to do the transformation we can ensure that all uses of the reflection API have the qctx.reflect import or explicit path. This delimits in a clearer way where the reflect API is used.

Migration

val expr: Expr[Any] = ...
- val term: Term = expr.unseal
- val term: Term = expr.asReflectTree
+ val term: Term = Term.of(expr)

- val tree: Tree = expr.unseal
- val tree: Tree = expr.asReflectTree
+ val tree: Tree = Tree.of(expr)

val exprList: List[Expr[Any]] = ...
- exprList.map(_.unseal)
- exprList.map(_.asReflectTree)
+ exprList.map(Term.of)

[liufengyun]

LGTM