Token claims

[MOZGIII]

This PR implements the token claiming logic.

See #430.

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To do

• basic implementation
• add pallet account to hold the funds
• check that the pallet account's balance is properly initialized at genesis build
• check that the total claimable balance is matching the expected value
• test claim call: success without vesting
• test claim call: success with vesting
• test claim call: eth signature recovery failure
• test claim call: eth signature recovers invalid eth address
• test claim call: locking under vesting fails (returns an error)
• test claim call: no claim exists
• test that we can consume all the claimable balance with claim calls, but not the existential balance of the pot
• bench: interface to runtime
• bench: implement test interface for mock runtime
• bench: claim call
• doc pallet
• doc claim call

[MOZGIII]

We'll be merging this soon, and moving on to building things on top of this PR. Please review asap.

[dmitrylavrenov]

Great! I was enjoyed walking through test cases. Looks like that we've covered all possible test cases for claim logic.

[intendednull]

LGTM!

[henrybyhee]

Looks good