Remove imperativity by moving all assertions to the end

[L-as]

The imperativity makes it hard to understand what is happening and what is being checked. All checks should be deferred to the end by checking that a list of booleans is all true.

[MartinOndejka]

Also in mina codebase I usually see monadic syntax for cheks, can we use that instead?

[L-as]

yes we should

[L-as]

Important for auditing reasons, is a minor amount of work.