Add optimiser docs

Author: andrejusk

docs/img/opt-19.png

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+# Optimiser
+
+The optimiser uses the syntax tree generated by the parser and prunes redundant
+parts of it. This greatly reduces the size of the program to be compiled, while achieving the
+same result.
+
+In this compiler, the optimisation runs before the target code generation, so the
+generated code is optimised. The implemented optimisations include:
+
+* Replacing constant variables with constants – this is done by checking if a variable
+is ever reassigned. If it is not reassigned (i.e. it is a constant), all references to the
+variable are replaced with the constant instead.
+
+* Replacing operations on two constants with the result – this is done by checking
+arithmetic expressions for having constants as both of the variables. If so, the
+operation gets performed during compilation-time.
+
+* Removing unused variables – this is done by checking if a variable is declared but
+never used. If this is the case, the declaration is removed.
+
+* Reducing the size of the tree – this is done by reducing the number of sequence
+nodes by merging sequence nodes with only one child.
+
+This is done by recursively optimising the syntax tree repeatedly until no more
+optimisations are made during a single run. As a result of this amount of heavy optimisation,
+all of the sample programs are compiled down to a single print statement.
+
+The optimised version of a previous example can be seen in
+Figure 19.
+
+![Figure 19](./img/opt-19.png)
+
+*Figure 19. Optimised Version of Tree in Figure 13 (A1.2. number-addition.ts)*
+
+If the target code generator is given the optimised syntax tree above, the code in
+Figure 20 is generated.
+
+```C
+ #include <stdio.h>
+ int main() {
+ printf("%d", 3);
+ return 0;
+ }
+```
+*Figure 20. Code Generated from Figure 19 (A1.2. number-addition.ts)*
+
+Finally, running the Mocha test suite on the optimised compiler reveals that all of the
+tests defined in the Test Cases section pass, as seen in Figure 21.
+
+![Figure 21](./img/opt-21.png)
+
+*Figure 21. Final Run of Mocha Test Suite (from Test Cases)*