Algebra: Fixed-point arithmetic

[Helco]

• Find a suitable library for fixed-point arithmetic
• Integrate with CMake behind compile constant
• Add tests to check precision, particular in higher-valued calculations (+/- 10000)
• Test renderer on pc and pebble
• Measure performance difference with #67

[Helco]

Research notes: libfixmath seems suitable:

• Q16.16 type (integer part +- 2^16, fractional part: 1 / (2 ^16))
• tested on ARM-Cortex M0
• seems easy to integrate
• has all of the necessary functions