Remove unnecessary branches

[jasonpaulos]

This PR adds additional code to TealBlock.NormalizeBlocks to remove unnecessary intermediate blocks with 0 ops. Blocks like this are added by If and other branching expressions in order to converge divergent branches. They are harmless, but it would be more efficient to get rid of them.

For example, consider the following expression:

If(Int(1)).Then(Int(2)).ElseIf(Int(3)).Then(Int(4)).Else(Int(5))

Before this PR, it compiles to:

#pragma version 2
int 1
bnz main_l5
int 3
bnz main_l4
int 5
main_l3:
b main_l6
main_l4:
int 4
b main_l3
main_l5:
int 2
main_l6:
return

With this PR, it now compiles to:

#pragma version 2
int 1
bnz main_l4
int 3
bnz main_l3
int 5
b main_l5
main_l3:
int 4
b main_l5
main_l4:
int 2
main_l5:
return

[jannotti]

How do we version this stuff? Since it changes compilation, logicsig authors may be sad. Do you only introduce these in new teal versions?

[jasonpaulos]

This change will get released with v0.9.0 of pyteal, which is allowed to introduce breaking changes. In the past we've changed the code generator on releases like this out of necessity (and always with a warning in the changelog), so there is precedence for this.

[algochoi]

Code looks good to me
Just had one question about manual list splicing.