Unify Spore factory methods

[jspenger]

This PR unifies the Spore factory methods.

Before, the factory methods consisted of: Spore.apply(body), Spore.applyWithEnv(env)(body), Spore.applyWithCtx(env)(body).

These are replaced by Spore(captures...)(body), where captures is either an explicit list of the captured variables, or the symbol * for capturing all variables.

Examples:

A Spore with no captures

Spore() { (x: Int) => x + 12 }

A Spore with explicit captures

val y = 12
val z = 13
Spore(y, z) { (x: Int) => x + y + z }

A Spore with * capture all mode

val y = 12
val z = 13
Spore(*) { (x: Int) => x + y + z }

Invalid capturing causes a compiler error.

val y = 12
val z = 13
Spore(y) { (x: Int) => x + y + z }
//                             ^
// Error: Invalid capture of variable `z`.
// Add it to the capture list or use `*` to capture all by default.

[jspenger]

Future improvement:
The API now consists of a single method. However, the new method's signature does not reflect what happens after macro expansion. For example, the signature hides using/implicit clauses. One option is to provide overloaded aliases with explicit signatures. Three cases would be useful: the case of an empty capture list; the case of one explicit capture and its evidence; and the case of the * capture all mode.