lang: Optimize sized fields when using LazyAccount

[acheroncrypto]

Problem

We currently read the account data and get the offset to get each field's size:

anchor/lang/attribute/account/src/lazy.rs

Lines 60 to 64 in 879601e

	let size = quote! {
	<#ty as anchor_lang::__private::Lazy>::size_of(
	&self.__info.data.borrow()[self.#offset_of_ident()..]
	)
	};

This is very inefficient and unnecessary if the type is sized.

Summary of changes

Add associated constant, SIZED, and use it to decide whether to read the account data and get the offset or not.

[vercel]

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