🏗 Merge C++ into python codebase (#504)

## Description:

Move esphome-core codebase into esphome (and a bunch of other refactors). See https://github.com/esphome/feature-requests/issues/97

Yes this is a shit ton of work and no there's no way to automate it :( But it will be worth it 👍

Progress:
- Core support (file copy etc): 80%
- Base Abstractions (light, switch): ~50%
- Integrations: ~10%
- Working? Yes, (but only with ported components).

Other refactors:
- Moves all codegen related stuff into a single class: `esphome.codegen` (imported as `cg`)
- Rework coroutine syntax
- Move from `component/platform.py` to `domain/component.py` structure as with HA
- Move all defaults out of C++ and into config validation.
- Remove `make_...` helpers from Application class. Reason: Merge conflicts with every single new integration.
- Pointer Variables are stored globally instead of locally in setup(). Reason: stack size limit.

Future work:
- Rework const.py - Move all `CONF_...` into a conf class (usage `conf.UPDATE_INTERVAL` vs `CONF_UPDATE_INTERVAL`). Reason: Less convoluted import block
- Enable loading from `custom_components` folder.

**Related issue (if applicable):** https://github.com/esphome/feature-requests/issues/97

**Pull request in [esphome-docs](https://github.com/esphome/esphome-docs) with documentation (if applicable):** esphome/esphome-docs#<esphome-docs PR number goes here>

## Checklist:
  - [ ] The code change is tested and works locally.
  - [ ] Tests have been added to verify that the new code works (under `tests/` folder).

If user exposed functionality or configuration variables are added/changed:
  - [ ] Documentation added/updated in [esphomedocs](https://github.com/OttoWinter/esphomedocs).

esphome/components/api/api_message.cpp

@@ -0,0 +1,87 @@
+#include "api_message.h"
+#include "esphome/core/log.h"
+
+namespace esphome {
+namespace api {
+
+static const char *TAG = "api.message";
+
+bool APIMessage::decode_varint(uint32_t field_id, uint32_t value) { return false; }
+bool APIMessage::decode_length_delimited(uint32_t field_id, const uint8_t *value, size_t len) { return false; }
+bool APIMessage::decode_32bit(uint32_t field_id, uint32_t value) { return false; }
+void APIMessage::encode(APIBuffer &buffer) {}
+void APIMessage::decode(const uint8_t *buffer, size_t length) {
+  uint32_t i = 0;
+  bool error = false;
+  while (i < length) {
+    uint32_t consumed;
+    auto res = proto_decode_varuint32(&buffer[i], length - i, &consumed);
+    if (!res.has_value()) {
+      ESP_LOGV(TAG, "Invalid field start at %u", i);
+      break;
+    }
+
+    uint32_t field_type = (*res) & 0b111;
+    uint32_t field_id = (*res) >> 3;
+    i += consumed;
+
+    switch (field_type) {
+      case 0: {  // VarInt
+        res = proto_decode_varuint32(&buffer[i], length - i, &consumed);
+        if (!res.has_value()) {
+          ESP_LOGV(TAG, "Invalid VarInt at %u", i);
+          error = true;
+          break;
+        }
+        if (!this->decode_varint(field_id, *res)) {
+          ESP_LOGV(TAG, "Cannot decode VarInt field %u with value %u!", field_id, *res);
+        }
+        i += consumed;
+        break;
+      }
+      case 2: {  // Length-delimited
+        res = proto_decode_varuint32(&buffer[i], length - i, &consumed);
+        if (!res.has_value()) {
+          ESP_LOGV(TAG, "Invalid Length Delimited at %u", i);
+          error = true;
+          break;
+        }
+        i += consumed;
+        if (*res > length - i) {
+          ESP_LOGV(TAG, "Out-of-bounds Length Delimited at %u", i);
+          error = true;
+          break;
+        }
+        if (!this->decode_length_delimited(field_id, &buffer[i], *res)) {
+          ESP_LOGV(TAG, "Cannot decode Length Delimited field %u!", field_id);
+        }
+        i += *res;
+        break;
+      }
+      case 5: {  // 32-bit
+        if (length - i < 4) {
+          ESP_LOGV(TAG, "Out-of-bounds Fixed32-bit at %u", i);
+          error = true;
+          break;
+        }
+        uint32_t val = (uint32_t(buffer[i]) << 0) | (uint32_t(buffer[i + 1]) << 8) | (uint32_t(buffer[i + 2]) << 16) |
+                       (uint32_t(buffer[i + 3]) << 24);
+        if (!this->decode_32bit(field_id, val)) {
+          ESP_LOGV(TAG, "Cannot decode 32-bit field %u with value %u!", field_id, val);
+        }
+        i += 4;
+        break;
+      }
+      default:
+        ESP_LOGV(TAG, "Invalid field type at %u", i);
+        error = true;
+        break;
+    }
+    if (error) {
+      break;
+    }
+  }
+}
+
+}  // namespace api
+}  // namespace esphome