refactor: remove read state encoding

[Daanvdplas]

Closes #101

• Removes the decoding of the read state result
• refactors the read state implementation by matching on the decoded key in the pallet in stead of the chain extension
• adds a filter for state queries (useful for the different runtimes)

Contract size from 7.4K to 7.1K

Created an issue to look into state query error handling #123

[evilrobot-01]

A few minor comments which need addressing.

[evilrobot-01]

• refactor: charging weight for state queries

If it will always be one or more reads, we could attach an attribute to the read enum variants which defines the number of reads. That way the extension can just use that static number to precharge the relevant weight, but more importantly it is defined in the pallet. Alternatively it could be an actual weight value, but that might be something for the future.

[evilrobot-01]

Looks good, has merge conflicts tho.