Handle QuoteContext paths explicitly

[nicolasstucki]

Provide a stronger invariant internal encoding for splices that depend on outer an outer QuoteContext

Currently we use

def exprSplice[T, QCtx <: QuoteContext](x: QCtx ?=> Expr[T]): T = ???

exprSplice[T, qctx.NestedContext](...)

assuming that QCtx will be a qctx.NestedContext if the splice has an outer context or QuoteContext if it does not. This encoding is quite fragile as QCtx could get widen and lose the path when typing.

Instead, we use the following encoding

def exprSplice[T](x: QuoteContext ?=> Expr[T]): T = ???
def exprNestedSplice[T](ctx: QuoteContext)(x: ctx.NestedContext ?=> Expr[T]): T = ???

exprSplice[T](ctx)(...)
exprNestedSplice[T](ctx)(...)

With this encoding, there is only one option for the type of the context function x.

[liufengyun]

LGTM

[nicolasstucki]

Let's wait until we check that nightlies are correctly published from GitHub actions.

[nicolasstucki]

Rebased. No changes.