`compile_prism` can take a source and file (and other arguments) or a
file as the source. `compile` checks if the source is a file and if it
is converts it. `compile_prism` is now doing the same thing.

On the Ruby side `compile` handles a file
[here](https://github.com/ruby/ruby/blob/master/iseq.c#L1159-L1162).

Before:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(26,21)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] name@0
0000 putstring                              "Prism"                   (  25)[Li]
0002 setlocal                               name@0, 0
0005 putself                                                          (  26)[Li]
0006 putobject                              "hello, "
0008 getlocal                               name@0, 0
0011 dup
0012 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0014 anytostring
0015 concatstrings                          2
0017 send                                   <calldata!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, nil
0020 leave
hello, Prism

"********* PRISM *************"
./test.rb:13:in `compile_prism': wrong argument type File (expected String) (TypeError)
	from ./test.rb:13:in `<main>'
make: *** [run] Error 1
```

After:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(26,21)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] name@0
0000 putstring                              "Prism"                   (  25)[Li]
0002 setlocal                               name@0, 0
0005 putself                                                          (  26)[Li]
0006 putobject                              "hello, "
0008 getlocal                               name@0, 0
0011 dup
0012 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0014 anytostring
0015 concatstrings                          2
0017 send                                   <calldata!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, nil
0020 leave

"********* PRISM *************"
== disasm: #<ISeq:<compiled>@test_code.rb:24 (24,0)-(25,21)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] name@0
0000 putstring                              "Prism"                   (  24)[Li]
0002 setlocal                               name@0, 0
0005 putself                                                          (  25)[Li]
0006 putobject                              "hello, "
0008 getlocal                               name@0, 0
0011 dup
0012 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0014 anytostring
0015 concatstrings                          2
0017 send                                   <calldata!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, nil
0020 leave                                                            (  24)
```

Fixes ruby/prism#1609