Feature: add an infrastructure-level SharesRequestor

[SeanJCasey]

Addresses issue #954

The purpose of this PR is to abstract the requesting of shares from Participation.sol into a new infrastructure-level SharesRequestor.sol.

In addition, it also:

• combines the remaining Participation code into a single Shares component
• fixes bug in isFund() that causes a generic revert if address is not a fund or spoke
• adds __increaseApproval() and __decreaseApproval() to TokenUser.sol, and cleans up that contract
• refactor remaining parts of Participation.sol, improving namespacing, comments, and readability

[SeanJCasey]

Further work to do here before finishing up:

• [x] refactor with EnumerableSet
• [x] add SharesRequestor unit tests
• [x] add Participation unit tests
• [ ] add checking if request passes fund policies to request validation step (?)
• [x] revisit / confirm RedeemSharesWithConstraints logic, esp fees
• [ ] consider what kinds of event return params are helpful to index (discuss with frontend)
• [x] update changelog
• [x] update arch changelog