Eager bitfield validation in parameters about builtin-actors

[lyswifter]

@Stebalien I wonder if I could just change the UnvalidateBitField to BitField to avoid repeated validating actions. I am a little confused about validating them in parameters and return values mean.

[Stebalien]

I mean, replace all instances of UnvalidatedBitField with BitField in actor method parameters/return values. E.g.

builtin-actors/actors/miner/src/types.rs

Lines 141 to 147 in bdc3f85

	#[derive(Serialize_tuple, Deserialize_tuple)]
	pub struct ExpirationExtension {
	pub deadline: u64,
	pub partition: u64,
	pub sectors: UnvalidatedBitField,
	pub new_expiration: ChainEpoch,
	}

Basically, any type taken/returned by a method called in an invoke implementation like

builtin-actors/actors/miner/src/lib.rs

Line 4742 in bdc3f85

fn invoke_method<BS, RT>(

.

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The rational is:

• Validate all external inputs.
• Avoid parsing/validating internal state on-load.