`chomp` uses `SubString` so `chomp!` is not needed to ensure that copying of big string is not performed.

`chomp!` was only used in `readchomp` and I propose to replace it with `chomp` there. As `chomp!` was not exported I understand that it can be removed without deprecation.

In this way copying of string is avoided, and in `strip` currently the string is actually copied twice. A basic benchmark is given below:
```
julia> using BenchmarkTools

julia> s = " "^10*"x"^1000*" "^10;

julia> @benchmark strip(s) # current definition
BenchmarkTools.Trial:
  memory estimate:  2.13 KiB
  allocs estimate:  2
  --------------
  minimum time:     502.056 ns (0.00% GC)
  median time:      537.579 ns (0.00% GC)
  mean time:        631.383 ns (5.74% GC)
  maximum time:     6.003 μs (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     197

julia> @benchmark strip(s) # with SubString definition
BenchmarkTools.Trial:
  memory estimate:  64 bytes
  allocs estimate:  2
  --------------
  minimum time:     334.062 ns (0.00% GC)
  median time:      339.819 ns (0.00% GC)
  mean time:        362.150 ns (1.00% GC)
  maximum time:     8.236 μs (93.19% GC)
  --------------
  samples:          10000
  evals/sample:     243
```