Improve leaving a result from statements

[pvginkel]

Programs and eval's return the result of the last executed expression. Currently this is implemented by requiring all expression statements to put the result in a local for the last statement of a program. This recurses into the statement, so if a statement contains other statements, those expression statements apply too. Currently, all expression statements in that tree will write their result into a local. However, this is not required. Only the last statement of a block and the last statement before break statements should be writing their results.