[#302] Fix integer conversion in `morley-client get-balance`

Description

Problem: morley-client get-balance was reporting wrong results for large balances due to an accidental conversion from Int64 to Word32, which truncated balances >= 4294 Tz.

Solution:

• The NodeAPI type has been changed to return a new TezosMutez type instead of TezosInt64. The conversion to Mutez is handled in the instance FromJSON TezosMutez.

• To avoid code duplication, parseMutezJson is replaced by relying on FromJSON TezosMutez.

Related issue(s)

Resolves #302 (closed).

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