Lift the `Default` bound for the `Circuit` trait

[ureeves]

The Compiler::compile function keeps this bound since it actually requires the circuit to have a Default implementation, but all other functionality now works without said bound.

Resolves #715

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Impacted Files	Coverage Δ
src/composer/compiler.rs	98.57% <ø> (ø)

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[vlopes11]

The Default trait is meant to be the definition in which the circuit will compile to. The existence of the execute circuit should not be taken as reference because it was created several plonk/rust versions ago, when the language limited for instance associated constants for traits (and a couple of other things).

The general idea behind this concept is that a type should have a single circuit (hence, a keys set). This will provide type-safety and will make it easier to store the keys statically.

[ureeves]

If it is the case that we shouldn't rely on downstream to inform us on what the API should look like, and we really want to enforce type safety - which is already questionable in the current implementation (see #713) - the we should remove Compiler::compile_with_circuit instead. I'm not sure how to proceed here. As I mention in the linked issue, the exact problem you mention this is intended to address happens anyway.

[vlopes11]

You mean that the user can serialize a PLONK key using a type A, and if he deserializes with type B this is a violation of type-safety?

If we feel this is indeed a violation to the principle, then my suggestion would be to fix that issue and improve the security instead of removing the concept. One alternative is to put additional bytes in the key to identify the type, and assert upon deserialization. But my feeling is this is overkill as it will consume more bytes.

The fact that we can't have this implementation for the execute circuit just highlights it should be refactored to comply with the new rules.