Migrate away from SimpleDispatchInfo

[kianenigma]

• Deprecates SimpleDispatchInfo, replaced with impls for Weight, (Weight, Class) and (Weight, Class, PaysFee)
• Revamps all the docs accordingly. Since the new weight system is being prepared and will be presented in Sub0, this is quite important, the old docs were very much out of date.

I brought this up (intention to deprecate SimpleDispatchInfo) and there were no objections, so I assume it is okay. SimpleDispatchInfo was verbose, not flexible (i.e. for each permutation of (weight, class, fee) you need a new enum variant) and does not have any added value. I always saw it as hasty mistake which was added in the first version of weights.

So all in all, If this is going to happen, better for it to happen now before 2.0.

This PR should change no logic except for in weight.rs. substitution regex:

// FixedNormal pays fee, default class is Normal, only replace weight
SimpleDispatchInfo::FixedNormal\((.*)\)
$1

// FixedOperational pays fee, no need to set third value
SimpleDispatchInfo::FixedOperational\((.*)\)
\($1, frame_support::weights::DispatchClass::Operational\)

// FixedMandatory pays fee, no need to set third value
SimpleDispatchInfo::FixedMandatory\((.*)\)
\($1, frame_support::weights::DispatchClass::Mandatory\)

• polkadot companion: Companion for substrate/5686 polkadot#1008
• clean some imports.

[kianenigma]

interesting case not automatically migrated.

[kianenigma]

Interesting case not handled by the regex.

[shawntabrizi]

LGTM when you create polkadot pr

[gnunicorn]

tests failing.

[kianenigma]

should be good now. @gnunicorn

[kianenigma]

Updated the polkadot PR as well and both should be mergeable now.