Implement global string functions

[sjbarag]

We've already got four of these, but it'd be great if we had a few more! Specifically, there's a bunch of [global string functions]() in BrightScript:

• Instr(start as Integer, text as String, substring as String) as Integer
• Left(s as String, n as Integer) as String
• Len(s as String) as Integer
• Mid(s as String, p as Integer, [n as Integer]) as String
• Right(s as String, n as Integer) as String
• Str(value as Float) as String
• StrI(value as Integer) as String
• StrI(value as Integer, radix as Integer) as String
• String(n as Integer, str as String ) as String
• StringI(n as Integer, ch as Integer) as String
• Val(s as String) as Float
• Val(str as String, radix as Integer) as Integer
• Substitute(str as String, arg0 as String, arg1 = "" as String, arg2 = "" as String, arg3 = "" as String) as String

[sjbarag]

Same deal as #43: I've marked this as "Good first issue" because we have a pretty good pattern around global functions now. Also note that -- because this is a first issue -- a first-time contributor doesn't have to implement all of these at once. That's a pretty big undertaking. We can always split this into multiple issues to handle families of string functions (e.g. "all the ones that pad strings")