Tests for X and some misc changes

[frankhart2018]

Implementation

1. Add test sub-packages for various sub-packages of X.
2. Make running test command with X installation.
3. Change representation of token type from string to TokenType (enum).
4. In webdebug /generate-tokens, the string representation of Token should be used, it should not build its own representation of Token.
5. Add string representation of Token and TokenType.
6. Update parser to account for the new TokenType enum.
7. Stricter mypy rules.
8. Add a class CheckClass which is inherited by all classes, this does input validation before constructing an object of a class.

[frankhart2018]

Changes are pending, created this PR so as to not forget listing out the changes in implementation. I will continue pushing to the same PR. This will be merged once all the changes for #35 are done.

[frankhart2018]

Status:

• [x] langx/tokenizer/token.py
• [x] langx/tokenizer/tokenizer.py
• [x] langx/tokenizer/token_type.py
• [x] langx/utils/error.py
• [x] langx/utils/tree_walker.py
• [x] langx/parser/parser.py
• [x] langx/parser/node/number_node.py
• [x] langx/parser/node/expr_node.py
• [x] langx/parser/node/print_node.py
• [x] langx/parser/node/binary_operator_node.py
• [x] langx/parser/node/program_node.py
• [x] langx/parser/node/string_node.py
• [x] langx/vm/vm.py
• [x] langx/decompiler/python/decompile.py
• [x] langx/decompiler/java/decompile.py
• [x] langx/decompiler/cpp/decompile.py
• [x] langx/decompiler/c/decompile.py
• [x] langx/table/symbol_table.py
• [x] langx/compiler/compiler.py
• [x] langx/opcode/opcode.py
• [x] langx/opcode/op_type.py

[frankhart2018]

TODO:

• [x] Add equality checks for nodes
• [x] Add null checks