[RFC] [Contrib] Minimal runtime (~12kb .text on ARMv7/x86) for subset…

… of TVM models

This is an alternative implementation of a subset of the TVM runtime API (and
graph runtime) that focuses entirely on reducing code size, at the expense of
functionality (no tvm.extern(..) calls via PackedFunc, CPU only, etc). It might
be worth incrementally expanding the surface area if there's interest.

The motivation for this work was seeing what the minimal useful subset of the
TVM runtime is. This is relevant for e.g. super code-size constrained
applications in e.g. embedded/mobile. The current runtime is more like O(100KiB)
or so, so this might be compelling for some users.

The smaller surface area for auditing might make this relevant for
#3159, or the usecases I was thinking about in
#2523 (comment) re: the Rust
runtime.

The symbols in the tvm::minimalruntime space (i.e. excluding std:: and
picojson::) are about 5KiB, so I think there's a bunch of room here (i.e. we
could replace picojson:: with [`jsmn`](https://zserge.com/jsmn.html) or
something, and we could replace more of the `std::unordered_map` usage, etc with
custom primitives as well (similar to the `DynArray`).

3rdparty/picojson/README.md

@@ -0,0 +1,20 @@
+<!--- Licensed to the Apache Software Foundation (ASF) under one -->
+<!--- or more contributor license agreements.  See the NOTICE file -->
+<!--- distributed with this work for additional information -->
+<!--- regarding copyright ownership.  The ASF licenses this file -->
+<!--- to you under the Apache License, Version 2.0 (the -->
+<!--- "License"); you may not use this file except in compliance -->
+<!--- with the License.  You may obtain a copy of the License at -->
+
+<!---   http://www.apache.org/licenses/LICENSE-2.0 -->
+
+<!--- Unless required by applicable law or agreed to in writing, -->
+<!--- software distributed under the License is distributed on an -->
+<!--- "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY -->
+<!--- KIND, either express or implied.  See the License for the -->
+<!--- specific language governing permissions and limitations -->
+<!--- under the License. -->
+
+## Notes
+
+`picojson.h` is derived from https://github.com/kazuho/picojson.