Split Project into Submodules

Overview
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I split the stdbot project into multiple submodules. Thus, modules are easier to develop, maintain, and remove as all their information is congregated into one place.


Modules
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The project now consists of the following submodules:

* `stdbot_file`

* `stdbot_graph`

* `stdbot_math`

* `stdbot_memory`

* `stdbot_sensors`

* `stdbot_timer`


Changes
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The main file, `stdbot.h`, now only contains `#include` statements and function prototypes. Thus, the end user can read `stdbot.h` in order to understand how to use the stdbot library without being overwhelmed with the library's implementation.

Each submodule's implementation is contained in a separate file.

As part of the separation, internal variables are prefixed with `__stdbot_<modulename>_`, to prevent names from colliding.

I also updated the `README.md` to reflect the structure changes.