Fix Nested Array

[Enmk]

Fixes

• Fixed ColumnArray::Slice() and ColumnArray::GetAsColumn() that worked poorly with 2D-arrays (Array(Array(X)))
  Previous implementation was using Append, rather than AppendAsColumn, flattening nested arrays.

New APIs

• Column::CloneEmpty - idiomatic way to make an empty clone of a column instead of c->Slice(0, 0)
• ColumnArray::GetAsColumnTyped<T> - syntax sugar to simplify accessing array values.
• ColumnArray<T> template that simplifies and optimizes both reading and appending data to Array.
  • For more optimal access to the Array's data, both ColumnArrayT<X>::operator[] and ColumnArrayT<X>::At() provide a wrapper rather than a full-blown column. So the underlying column is not copied, but rather accessed via read-only "view".
  • ColumnArrayT<X>::Append() overloads work for anything iteratable. Users don't have to wrap values into ColumnX anymore.
  • For convenience, values obtained from both ColumnArrayT<X>::operator[] and ColumnArrayT<X>::At() provide type-safe access to the typed data.
  • Existing ColumnArray instance can be type-safe wrapped into appropriate ColumnArrayT<NestedColumnType> so users can benefit from features above for columns obtained from Block.

Examples

• Array(UInt64):

auto array = std::make_shared<clickhouse::ColumnArrayT<ColumnUInt64>>();
array->Append({0, 1, 2});

EXPECT_EQ(0u, array->At(0).At(0)); // 0
EXPECT_EQ(1u, (*array)[0][1]);     // 1

• Array(FixedString(6)):

using namespace std::literals;
auto array = std::make_shared<clickhouse::ColumnArrayT<ColumnFixedString>>(6); // argument is passed to ColumnFixedString constructor
array->Append({"hello", "world"});

// Additional \0 since strings are padded from right with zeros in FixedString(6).
EXPECT_EQ("hello\0"sv, array->At(0).At(0));

auto row = array->At(0);
EXPECT_EQ("hello\0"sv, row.At(0));
EXPECT_EQ(6u, row[0].length());
EXPECT_EQ("hello", row[0].substr(0, 5));

EXPECT_EQ("world\0"sv, (*array)[0][1]);

• Array(Array(UInt64))

// Nested 2D-arrays are supported too:
auto array = std::make_shared<clickhouse::ColumnArrayT<clickhouse::ColumnArrayT<ColumnUInt64>>>();
array->Append(std::vector<std::vector<unsigned int>>{{0}, {1, 1}, {2, 2, 2}});

EXPECT_EQ(0u, array->At(0).At(0).At(0)); // 0
EXPECT_EQ(1u, (*array)[0][1][1]);        // 1

See unit-tests for more examples.

Closes: #43

[arthurpassos]

I still need to go over some details and the tests. Submeting partial review so we can work in parallel.