feat(functions): return mutatble reference from add_block

[garritfra]

Fixes #17

Description

Function::add_block() should return a reference to the created block.

Changes proposed in this pull request

• Function::add_block() returns a reference to the created block
• Deprecated Function::last_block(), since it's mostly unnecessary

ToDo

• Proposed feature/fix is sufficiently tested
• Proposed feature/fix is sufficiently documented
• The "Unreleased" section in the changelog has been updated, if applicable

[YerinAlexey]

Function::last_block is useful for checking if the function always returns, for example

[garritfra]

Function::last_block is useful for checking if the function always returns, for example

But that can also be done via self.blocks.last(), which is hardly any more code to write.