Memoize Spree::User#wallet method

[AlessioRocco]

Description

We are already using memoization on some Spree::Order methods:

• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/order.rb#L263-L265
• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/order.rb#L294-L296
• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/order.rb#L298-L300
• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/order.rb#L302-L304

and Spree::User#wallet seems a good candidate to memoize.

Without memoization every time someone calls this method a new instance of Spree::Wallet is created but I think we should interact ever with the same instance, It seems logical to me that a user has just one wallet, this way the code reflect the reality and It also improves performance and memory usage.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[spaghetticode]

@AlessioRocco seems legit, thank you 👍

[kennyadsl]

@AlessioRocco I don't get why this is legit, can you expand a little bit more. Also, it would be helpful to understand what's wrong with the current version.

[AlessioRocco]

@kennyadsl I've updated the description to better explain because I did this change.

[kennyadsl]

Thanks Alessio!