Improve string ergonomics

[x87]

• [x] string literals (both single and double quoted) should support escape sequences. currently only strings inside hex..end do that:

hex
  "\t" // compiles as 09 (code for a TAB character)
end

0@v = "\t" // compiles as 5C 74 ("\t")

• [x] strings should support multiple spaces without escaping

hex
  "\t     \t" // leave one space between \t
end

0@v = "\t     \t" // same

• [x] SB3_compat mode