Move Staging to Typer

[nicolasstucki]

The main motivation for moving staging to typer is to support whitebox macros.

Using the <: T return type notation.

/** Returns a Char if the string is of length 1 or a String otherwise */
inline def charOrString(inline str: String) <: Char | String = ${ impl(str) }
def impl(str: String) = if (str.length == 1) str.charAt(0).toExpr else str.toExpr
// Different file
val x: Char = charOrString("1")
val y: String = charOrString("123")

Modification:

• Staging is used in Typer to check contents of inline macros
• Expand macros in Inliner durring typing
• Keep track of quotation level in the context
• Lose support for inspection of any tree in the current compilation unit. With the exception of parameter bindings which can be inspected.

[nicolasstucki]

Needs rebase on #5940 and remove 08a391d

[OlivierBlanvillain]

@nicolasstucki Can you explain the motivation for this change?

[nicolasstucki]

Mainly to support whitebox macros

[odersky]

I only have one more comment on the last commit. Otherwise it's good.

[nicolasstucki]

Rebased