Add select expression without query support

[dai-chen]

Issue #, if available: #545, #264, #256

Description of changes: Recently a new SQL module was added and integrate with new query engine. As first feature, select literal without query (FROM clause) was supported. In this PR, support for more expression selected without query is added to lay the foundation for future SQL functions work. In particular, the changes include:

1. Support select arithmetic expression without query: SELECT 1 + 2
2. Support select function expression without query: SELECT abs(-1)

Out of scope: Only one function abs is added in this PR as a sample. More functions and other expression such as predicate expression will be covered in future PR separately.

Testing: As comparison test framework is enable by default, new test convention is followed:

1. Comparison test: add test cases in arithmetics.txt and functions.txt
2. Edge case test: add ExpressionIT to test edge case like divide by zero

Documentation: Add one doc in doctest https://github.com/dai-chen/sql/blob/add-select-expression-support/docs/user/dql/expressions.rst#arithmetic-expressions.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[penghuo]

LGTM