String literal hash keys can't be mutated by the user so we should mark
them as frozen. We were seeing instructions for hashes with string
literal keys using two `putstring` instructions when it should be a
`putobject` and `putstring`.
Code example:
```ruby
{ "a" => "b" }
```
Instructions before:
```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,14)>
0000 putobject "a" ( 2)[Li]
0002 putstring "b"
0004 newhash 2
0006 leave
"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,14)>
0000 putstring "a" ( 1)[Li]
0002 putstring "b"
0004 newhash 2
0006 leave
```
Instructions after:
```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,14)>
0000 putobject "a" ( 2)[Li]
0002 putstring "b"
0004 newhash 2
0006 leave
"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,14)>
0000 putobject "a" ( 1)[Li]
0002 putstring "b"
0004 newhash 2
0006 leave
```
https://github.com/ruby/prism/commit/b14ae55385