Plumb deadline timer into mod_exp implementation

[arhag]

Moved from VaultaFoundation/mandel#785:

This allows us to remove the subjective limitation on bit size introduced with VaultaFoundation/mandel#361 since the deadline timer would be able to interrupt the computation done internally for mod_exp.

Requires code changes to be made to the math implementation of mod_exp which must wait until we replace that implementation from gmp to something with a friendlier license like BoringSSL (captured in VaultaFoundation/mandel#769).

Dependencies:

• Replace OpenSSL usage with BoringSSL leap#644
• Migrate mod_exp to BoringSSL, removing GMP requirement #1857