Rename `parse_string_unsafe` to `parse_string_liberally` so it's less intimidating

[timlnx]

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In the 2.0 update bug #99, it was brought up that parse_string_unsafe can have negative side effects

Using an "unsafe" function will be flagged in every code review and will cause the reviewer to dig into the implementation to verify that this is really "safe" to use

Originally posted by @FelixSchwarz in https://github.com/tbielawa/bitmath/issues/99#issuecomment-1427000623

I'm proposing _liberally because definition no. 2 when I looked it up is:

in a way that is not precise or strictly literal; loosely.

which feels accurate for what it does.

Thoughts?

[timlnx]

I suppose there are other ways to approach this.

Get rid of the _unsafe / _liberally functions completely and add a kwarg to the standard parse_string function which allows it to apply the less strict parsing logic.

[FelixSchwarz]

I never saw "liberally" in an API. Personally I sometimes use "guess_...". However I like something like a strict=True/False parameter even more. The default would be strict parsing but users could use a more "liberal" one.