Add utf8.lower & utf8.upper

[Be1zebub]

Implementation based on mapping.
This is compromise - performance and simplicity, in exchange for 48kb mem.
If this is unacceptable, i suggest doing the implementation using native functions - but this is beyond the scope of lua repo.

[CornerPin]

Unicode case mappings are not 1:1, one example is the Greek capital sigma Σ which has two lowercase forms σ and ς, the one that's chosen depends on if it's at the end of a word or not (5.18.2 Complications for Case Mapping).

When building uc_lc, conflicting characters will be overwritten in an undefined order. Here's a list of such characters that are in lc_uc:

Φ (U+03A6) -> φ (U+03C6) | ϕ (U+03D5)
Ρ (U+03A1) -> ρ (U+03C1) | ϱ (U+03F1)
I (U+0049) -> i (U+0069) | ı (U+0131)
S (U+0053) -> ſ (U+017F) | s (U+0073)
NJ (U+01CA) -> nj (U+01CC) | Nj (U+01CB)
Ε (U+0395) -> ϵ (U+03F5) | ε (U+03B5)
DZ (U+01F1) -> dz (U+01F3) | Dz (U+01F2)
Ṡ (U+1E60) -> ṡ (U+1E61) | ẛ (U+1E9B)
Β (U+0392) -> β (U+03B2) | ϐ (U+03D0)
Μ (U+039C) -> µ (U+00B5) | μ (U+03BC)
Κ (U+039A) -> ϰ (U+03F0) | κ (U+03BA)
Θ (U+0398) -> θ (U+03B8) | ϑ (U+03D1)
Σ (U+03A3) -> ς (U+03C2) | σ (U+03C3)
Ι (U+0399) -> ͅ (U+0345) | ι (U+1FBE) | ι (U+03B9)
LJ (U+01C7) -> Lj (U+01C8) | lj (U+01C9)
DŽ (U+01C4) -> dž (U+01C6) | Dž (U+01C5)
Π (U+03A0) -> π (U+03C0) | ϖ (U+03D6)

The Unicode Standard provides files such as UnicodeData.txt and SpecialCasing.txt that are relevant to these two functions (4.2 Case). The former defines simple 1:1 context-independent case mappings while the latter defines special mappings that require additional context.

Perhaps a better way to handle this would be something like

• utf8.lower(str, locale=nil)
• utf8.upper(str, locale=nil)

where locale=nil will only perform simple mappings? This is how ICU does it.

[Be1zebub]

@CornerPin
I dont think it plays a big role.
Just looked at the godot code and tested few other projects, it seems no one cares about special casing.
It also goes beyond a simple and effective implementation, better to make os bindings in this case.

ps: check php, they tried to implement this - but their code is incredibly slow and this feature doesnt even work - i think this is a good proof why we shouldnt implement this.