[Zoe] Add Zoe method that takes an invitation and returns installation, instance, instanceRecord etc.

[katelynsills]

What is the Problem Being Solved?

this is modeled on getInvitationFields(), currently in zoeTestHelpers. The observation is that clients outside the contracts often have an invitation and want access to verified value of various fields. getInvitationFields() hides the way that it's retrieved from the inviteIssuer. To make this easier to use, we should add methods to zoe that return the info that Zoe guarantees to be present in the invitation.

Description of the Design
instead of

export const getInvitationFields = (invitationIssuer, invitationP) =>
  E(invitationIssuer)
    .getAmountOf(invitationP)
    .then(amount => amount.value[0]);

zoe can synchronoously access the individual fields.

Security Considerations

The only security consequences are that clients of the API will be less aware of the invitation issuer's role.

Test Plan

None.

[erights]

See #1414 (comment)

[katelynsills]

Closed by #1414