impl Zeroize for BoxedMontyParams

[baloo]

The intent is to add support for zeroize on RSA's precomputed p_params and q_params.

BoxedMontyParams got placed in an Arc in #1143 but I'm not sure I follow the reason why.

To implement Zeroize, this changes remove the Arc. The content is still placed on the heap (There is an underlying BoxedUint).

[baloo]

Let's make that a draft until we can figure out the reasoning behind Arc.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.05%. Comparing base (0bd622e) to head (d97f947).

Files with missing lines	Patch %	Lines
src/modular/monty_params.rs	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1338      +/-   ##
==========================================
- Coverage   91.06%   91.05%   -0.02%     
==========================================
  Files         189      189              
  Lines       22654    22651       -3     
==========================================
- Hits        20630    20624       -6     
- Misses       2024     2027       +3     

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[tarcieri]

Arc prevents the params from being copied every single time a new value is made, incurring a heap allocation for each one. It would incur copies and significantly increase the amount of memory required.

That said we do generally try to make use of in-place operations to avoid heap allocations whenever possible.

See also previous discussion of this in #708

[tarcieri]

A potential alternative here could be to expose the equivalent of Arc::into_inner and/or Arc::get_mut for the params, namely BoxedMontyParams is a wrapper for the generic MontyParams.

Someone wanting to write a Zeroize impl as in RSA could do e.g. BoxedMontyParams::get_mut and obtain an Option<&mut MontyParams<_>>, and if they have exclusive access can now use the Zeroize impl on MontyParams.

[tarcieri]

This defeats the point of having a newtype at all. If we're not using the Arc then this should just be:

Suggested change

	pub struct BoxedMontyParams(MontyParams<crate::uint::boxed::BoxedUint>);
	pub type BoxedMontyParams = MontyParams<crate::uint::boxed::BoxedUint>;