fix: sync max multi_a pubkey with core

[ChrisCho-H]

Although this is checked indirectly by checking MAX_STACK_SIZE, it'd be better to check in threshold explicitly as core does.

[apoelstra]

o.O I had no idea this limit existed.

Great find!

[apoelstra]

A few extra thoughts:

• The "max stack size" limit is enforced during sanity checking (and can therefore be disabled and eventually adjusted) while the threshold limits are enforced during parsing and in the Rust type system (cannot be adjusted)
• The threshold limits also provide a hard barrier on how much memory an individual Miniscript node can take and I'd like them to be as small as possible. (Though this is a bit silly since we have no limit on the thresh combinator...maybe we should, on the basis that and ADD each take at least one byte and each child takes at least one byte, it could be limited to 2MB? Not sure that a 2-meg limit is much more useful than no limit at all.)

[apoelstra]

ACK 96d8662 successfully ran local tests

[apoelstra]

Will let @sanket1729 take a quick look before merging.

[sanket1729]

ACK 96d8662