Support the 'method' pseudo op

[fergalwalsh]

This PR provides a mechanism for pseudo ops to be supported in Tealish. So far only method is added.
I'll repeat a comment from the code here:

Note: Other pseudo ops like addr and base32 are not included here because their syntax isn't parseable by Tealish currently.
e.g addr(RIKLQ5HEVXAOAWYSW2LGQFYGWVO4J6LIAQQ72ZRULHZ4KS5NRPCCKYPCUU) is not parseable because the address isn't quoted.

In Teal this is written as

addr RIKLQ5HEVXAOAWYSW2LGQFYGWVO4J6LIAQQ72ZRULHZ4KS5NRPCCKYPCUU

No quotes! I'm not sure we'll ever support those directly as making the parser accept unquoted strings seems like a messy thing to do. We may just add some explicit helper functions to Tealish for these if necessary.

So in summary the only supported pseudo-op will be method for now.

[barnjamin]

For addr type, maybe something like in #60 where the 0x prefix is used to denote what follows is hex bytes.

edit:
followed up with an attempt at this #61

[gokselcoban]

It works well as method("name(uint64,uint64"), thank you.

On the other hand method pseudo-op doesn't work with const as expected.

It generates method CONST_NAME instead of method CONST_VALUE lines.

[fergalwalsh]

On the other hand method pseudo-op doesn't work with const as expected.

Thanks for reporting this. It's messy to solve at the moment but should be easier after an in-progress refactor so I'll leave as is for now. I added an expected failure test to track this for now.