**Problem**
Slash syntax is currently implemented via a series of implicit
converters (Conversion), which is not nice, partly because
the behavior is difficult to follow.
**Solution**
This removes all the implicit converters and moves the slashes
into:
1. / methods under Reference for subproject scoping.
2. / methods under ScopeAxis with Reference type constraint
for the initial Zero scoping.
3. Return RefThenConfig structure for intermediate config scoping.
4. / method under `Scoped`, which is base trait for the keys
to implement task scoping e.g. `compile / scalacOptions`.
5. Extension methods for ConfigKey.
6. Extension methods for AttributeKey.
Settings0 used to be a Map[Scope, AttributeMap], and is now a
Map[ScopedKey[x], x].
This is better because we don't need to decompose all ScopedKey[x]
into a Scope and an AttributeKey[x], for recomposing it back later,
which duplicates all ScopedKey[x]. It reduces the number of long-living
ScopedKey[x] by 8%, and the total number of instances by 1.4%.
Also it improves the performance of Settings0, which was responsible of
2.95% of the total CPU time, and is now responsible of 0.41%.
Problem
-------
By Typer phase it seems like lazy val is expanded out to
`def x(): Int` and synthetic `val x$lzy1`,
which results in NoSuchMethod `x$lzy1`.
Solution
--------
Assume the naming convention and grab the part before `$`.