Add some Hash's methods to BCDD::Context

[serradura]

Add some Hash's methods to BCDD::Context. They are:

• #slice to extract only the desired keys.
• #[], #dig, #fetch to access the values.
• #values_at and #fetch_values to get the values of the desired keys.

result = BCDD::Context::Success(:ok, a: 1, b: 2, c: {d: 4})

result[:a]         # 1
result.fetch(:a)   # 1
result.dig(:c, :d) # 4

result.slice(:a, :b) # {:a=>1, :b=>2}

result.values_at(:a, :b) # [1, 2]
result.fetch_values(:a, :b) # [1, 2]

These methods are available for BCDD::Context::Success and BCDD::Context::Failure instances.