Minor code cleanup

Merge pull request stan-dev#245 from stan-dev/sem-check-invariant-test

Add check to semantic check that original AST is not modified (only decorated)

Try with no fast math

Merge pull request stan-dev#144 from stan-dev/code-gen9

Fix stan-dev#63, which fixed low_dim_gauss_mix

Fix Windows bin already exists thing

Merge pull request stan-dev#123 from stan-dev/ode33-patch-1

minor doc fix