Make a couple of things 'const'

[ac000]

• tstr: Constify the 'str' parameter to nxt_tstr_compile()

Just as the commit subject says, this constifies the str parameter to nxt_tstr_compile()

I have some code like

    static nxt_str_t        accept_enc_str =                                    
                                    nxt_string("$header_accept_encoding");      
    static const nxt_str_t  comps_str = nxt_string("compressors");              
    static const nxt_str_t  mimes_str = nxt_string("types");

Looking at it, you may wonder why the first one isn't static const, well it can be after this patch...

• wasm: Add a missing 'const' qualifier in nxt_wasm_setup()

filesystem_str can be defined as static const rather than just static