Typevisitor not collecting constraints correctly

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There is an issue with the type visitor and how it is handling visiting statements. This causes a `NoneType` to try to be concatenated to a string. The issue arises when parsing the following snippet:

````
module mod
stationary stat
manifest aaa
manifest bbb
manifest ccc
manifest ddd

functions:

instructions:

x = 0
if ( 3 == 3) {
x = 3
} else {
x = 5
}
y = x
````

Using the following execution flags:
`-d -i resources/programs/scratch_pad.bs -p ./resources/ -t inkwell`

Reproduce the failure with the provided BioScript snippet and the `-d -i resources/programs/scratch_pad.bs -p ./resources/ -t inkwell` flags. Inspect the type visitor's statement handling and constraint collection; done means the example parses without a `NoneType` string-concatenation error and collects its constraints correctly.

Written by the indexing model from the issue text.